summaryrefslogtreecommitdiffstats
path: root/doc/user/command-line.xml
blob: 4b75eba915e40f7fc12c9bba667c0121227cf911 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
  KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE
  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <para>

  &SCons; provides a number of ways
  for the writer of the &SConscript; files
  to give the users who will run &SCons;
  a great deal of control over the build execution.
  The arguments that the user can specify on
  the command line are broken down into three types:

  </para>

  <variablelist>

    <varlistentry>
    <term>Options</term>

    <listitem>
    <para>

    Command-line options always begin with
    one or two <literal>-</literal> (hyphen) characters.
    &SCons; provides ways for you to examine
    and set options values from within your &SConscript; files,
    as well as the ability to define your own
    custom options.
    See <xref linkend="sect-command-line-options"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term>Variables</term>

    <listitem>
    <para>

    Any command-line argument containing an <literal>=</literal>
    (equal sign) is considered a variable setting with the form
    <varname>variable</varname>=<varname>value</varname>
    &SCons; provides direct access to
    all of the command-line variable settings,
    the ability to apply command-line variable settings
    to construction environments,
    and functions for configuring 
    specific types of variables
    (Boolean values, path names, etc.)
    with automatic validation of the user's specified values.
    See <xref linkend="sect-command-line-variables"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term>Targets</term>

    <listitem>
    <para>

    Any command-line argument that is not an option
    or a variable setting
    (does not begin with a hyphen
    and does not contain an equal sign)
    is considered a target that the user
    (presumably) wants &SCons; to build.
    A list of Node objects representing
    the target or targets to build.
    &SCons; provides access to the list of specified targets,
    as well as ways to set the default list of targets
    from within the &SConscript; files.
    See <xref linkend="sect-command-line-targets"></xref>, below.

    </para>
    </listitem>
    </varlistentry>

  </variablelist>

  <section id="sect-command-line-options">
  <title>Command-Line Options</title>

    <para>

    &SCons; has many <emphasis>command-line options</emphasis>
    that control its behavior.
    A &SCons; <emphasis>command-line option</emphasis>
    always begins with one or two <literal>-</literal> (hyphen)
    characters.

    </para>

    <section>
    <title>Not Having to Specify Command-Line Options Each Time:  the &SCONSFLAGS; Environment Variable</title>

      <para>

      Users may find themselves supplying
      the same command-line options every time
      they run &SCons;.
      For example, you might find it saves time
      to specify a value of <literal>-j 2</literal>
      to have &SCons; run up to two build commands in parallel.
      To avoid having to type <literal>-j 2</literal> by hand
      every time,
      you can set the external environment variable
      &SCONSFLAGS; to a string containing
      command-line options that you want &SCons; to use.

      </para>

      <para>

      If, for example,
      you're using a POSIX shell that's
      compatible with the Bourne shell,
      and you always want &SCons; to use the
      <literal>-Q</literal> option,
      you can set the &SCONSFLAGS;
      environment as follows:

      </para>

      

      <screen>
        % <userinput>scons</userinput>
        scons: Reading SConscript files ...
        scons: done reading SConscript files.
        scons: Building targets ...
            ... [build output] ...
        scons: done building targets.
        % <userinput>export SCONSFLAGS="-Q"</userinput>
        % <userinput>scons</userinput>
            ... [build output] ...
      </screen>

      <para>

      Users of &csh;-style shells on POSIX systems
      can set the &SCONSFLAGS; environment as follows:

      </para>

      <screen>
        $ <userinput>setenv SCONSFLAGS "-Q"</userinput>
      </screen>

      <para>

      Windows users may typically want to set the
      &SCONSFLAGS; in the appropriate tab of the
      <literal>System Properties</literal> window.

      </para>

    </section>

    <section>
    <title>Getting Values Set by Command-Line Options:  the &GetOption; Function</title>

      <para>

      &SCons; provides the &GetOption; function
      to get the values set by the various command-line options.
      One common use of this is to check whether or not
      the <literal>-h</literal> or <literal>--help</literal> option
      has been specified.
      Normally, &SCons; does not print its help text
      until after it has read all of the &SConscript; files,
      because it's possible that help text has been added
      by some subsidiary &SConscript; file deep in the
      source tree hierarchy.
      Of course, reading all of the &SConscript; files
      takes extra time.

      </para>

      <para>

      If you know that your configuration does not define
      any additional help text in subsidiary &SConscript; files,
      you can speed up the command-line help available to users
      by using the &GetOption; function to load the
      subsidiary &SConscript; files only if the
      the user has <emphasis>not</emphasis> specified
      the <literal>-h</literal> or <literal>--help</literal> option,
      like so:

      </para>

      <programlisting></programlisting>

      <para>

      In general, the string that you pass to the
      &GetOption; function to fetch the value of a command-line
      option setting is the same as the "most common" long option name
      (beginning with two hyphen characters),
      although there are some exceptions.
      The list of &SCons; command-line options
      and the &GetOption; strings for fetching them,
      are available in the
      <xref linkend="sect-command-line-option-strings"></xref> section,
      below.

      </para>

    </section>

    <section>
    <title>Setting Values of Command-Line Options:  the &SetOption; Function</title>

      <para>

      You can also set the values of &SCons;
      command-line options from within the &SConscript; files
      by using the &SetOption; function.
      The strings that you use to set the values of &SCons;
      command-line options are available in the
      <xref linkend="sect-command-line-option-strings"></xref> section,
      below.

      </para>

      <para>

      One use of the &SetOption; function is to
      specify a value for the <literal>-j</literal>
      or <literal>--jobs</literal> option,
      so that users get the improved performance
      of a parallel build without having to specify the option by hand.
      A complicating factor is that a good value
      for the <literal>-j</literal> option is
      somewhat system-dependent.
      One rough guideline is that the more processors
      your system has,
      the higher you want to set the
      <literal>-j</literal> value,
      in order to take advantage of the number of CPUs.

      </para>

      <para>

      For example, suppose the administrators
      of your development systems
      have standardized on setting a
      <varname>NUM_CPU</varname> environment variable
      to the number of processors on each system.
      A little bit of Python code
      to access the environment variable
      and the &SetOption; function
      provide the right level of flexibility:

      </para>

      <programlisting>
        import os
        num_cpu = int(os.environ.get('NUM_CPU', 2))
        SetOption('num_jobs', num_cpu)
        print "running with -j", GetOption('num_jobs')
      </programlisting>

      <para>

      The above snippet of code
      sets the value of the <literal>--jobs</literal> option
      to the value specified in the
      <varname>$NUM_CPU</varname> environment variable.
      (This is one of the exception cases
      where the string is spelled differently from
      the from command-line option.
      The string for fetching or setting the <literal>--jobs</literal>
      value is <literal>num_jobs</literal>
      for historical reasons.)
      The code in this example prints the <literal>num_jobs</literal>
      value for illustrative purposes.
      It uses a default value of <literal>2</literal>
      to provide some minimal parallelism even on
      single-processor systems:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        running with -j 2
        scons: `.' is up to date.
      </screen>

      <para>

      But if the <varname>$NUM_CPU</varname>
      environment variable is set,
      then we use that for the default number of jobs:

      </para>

      <screen>
        % <userinput>export NUM_CPU="4"</userinput>
        % <userinput>scons -Q</userinput>
        running with -j 4
        scons: `.' is up to date.
      </screen>

      <para>

      But any explicit
      <literal>-j</literal> or <literal>--jobs</literal>
      value the user specifies an the command line is used first,
      regardless of whether or not
      the <varname>$NUM_CPU</varname> environment
      variable is set:

      </para>

      <screen>
        % <userinput>scons -Q -j 7</userinput>
        running with -j 7
        scons: `.' is up to date.
        % <userinput>export NUM_CPU="4"</userinput>
        % <userinput>scons -Q -j 3</userinput>
        running with -j 3
        scons: `.' is up to date.
      </screen>

    </section>

    <section id="sect-command-line-option-strings">
    <title>Strings for Getting or Setting Values of &SCons; Command-Line Options</title>

      <para>

      The strings that you can pass to the &GetOption;
      and &SetOption; functions usually correspond to the
      first long-form option name
      (beginning with two hyphen characters:  <literal>--</literal>),
      after replacing any remaining hyphen characters
      with underscores.

      </para>

      <para>

      The full list of strings and the variables they
      correspond to is as follows:

      </para>

      <informaltable>
      <tgroup cols="2" align="left">

      <thead>

      <row>
      <entry>String for &GetOption; and &SetOption;</entry>
      <entry>Command-Line Option(s)</entry>
      </row>

      </thead>

      <tbody>

      <row>
      <entry><literal>cache_debug</literal></entry>
      <entry><option>--cache-debug</option></entry>
      </row>

      <row>
      <entry><literal>cache_disable</literal></entry>
      <entry><option>--cache-disable</option></entry>
      </row>

      <row>
      <entry><literal>cache_force</literal></entry>
      <entry><option>--cache-force</option></entry>
      </row>

      <row>
      <entry><literal>cache_show</literal></entry>
      <entry><option>--cache-show</option></entry>
      </row>

      <row>
      <entry><literal>clean</literal></entry>
      <entry><option>-c</option>,
           <option>--clean</option>,
           <option>--remove</option></entry>
      </row>

      <row>
      <entry><literal>config</literal></entry>
      <entry><option>--config</option></entry>
      </row>

      <row>
      <entry><literal>directory</literal></entry>
      <entry><option>-C</option>,
             <option>--directory</option></entry>
      </row>

      <row>
      <entry><literal>diskcheck</literal></entry>
      <entry><option>--diskcheck</option></entry>
      </row>

      <row>
      <entry><literal>duplicate</literal></entry>
      <entry><option>--duplicate</option></entry>
      </row>

      <row>
      <entry><literal>file</literal></entry>
      <entry><option>-f</option>,
             <option>--file</option>,
             <option>--makefile </option>,
             <option>--sconstruct</option></entry>
      </row>

      <row>
      <entry><literal>help</literal></entry>
      <entry><option>-h</option>,
             <option>--help</option></entry>
      </row>

      <row>
      <entry><literal>ignore_errors</literal></entry>
      <entry><option>--ignore-errors</option></entry>
      </row>

      <row>
      <entry><literal>implicit_cache</literal></entry>
      <entry><option>--implicit-cache</option></entry>
      </row>

      <row>
      <entry><literal>implicit_deps_changed</literal></entry>
      <entry><option>--implicit-deps-changed</option></entry>
      </row>

      <row>
      <entry><literal>implicit_deps_unchanged</literal></entry>
      <entry><option>--implicit-deps-unchanged</option></entry>
      </row>

      <row>
      <entry><literal>interactive</literal></entry>
      <entry><option>--interact</option>,
             <option>--interactive</option></entry>
      </row>

      <row>
      <entry><literal>keep_going</literal></entry>
      <entry><option>-k</option>,
             <option>--keep-going</option></entry>
      </row>

      <row>
      <entry><literal>max_drift</literal></entry>
      <entry><option>--max-drift</option></entry>
      </row>

      <row>
      <entry><literal>no_exec</literal></entry>
      <entry><option>-n</option>,
             <option>--no-exec</option>,
             <option>--just-print</option>,
             <option>--dry-run</option>,
             <option>--recon</option></entry>
      </row>

      <row>
      <entry><literal>no_site_dir</literal></entry>
      <entry><option>--no-site-dir</option></entry>
      </row>

      <row>
      <entry><literal>num_jobs</literal></entry>
      <entry><option>-j</option>,
             <option>--jobs</option></entry>
      </row>

      <row>
      <entry><literal>profile_file</literal></entry>
      <entry><option>--profile</option></entry>
      </row>

      <row>
      <entry><literal>question</literal></entry>
      <entry><option>-q</option>,
             <option>--question</option></entry>
      </row>

      <row>
      <entry><literal>random</literal></entry>
      <entry><option>--random</option></entry>
      </row>

      <row>
      <entry><literal>repository</literal></entry>
      <entry><option>-Y</option>,
             <option>--repository</option>,
             <option>--srcdir</option></entry>
      </row>

      <row>
      <entry><literal>silent</literal></entry>
      <entry><option>-s</option>,
             <option>--silent</option>,
             <option>--quiet</option></entry>
      </row>

      <row>
      <entry><literal>site_dir</literal></entry>
      <entry><option>--site-dir</option></entry>
      </row>

      <row>
      <entry><literal>stack_size</literal></entry>
      <entry><option>--stack-size</option></entry>
      </row>

      <row>
      <entry><literal>taskmastertrace_file</literal></entry>
      <entry><option>--taskmastertrace</option></entry>
      </row>

      <row>
      <entry><literal>warn</literal></entry>
      <entry><option>--warn</option> <option>--warning</option></entry>
      </row>

      </tbody>

      </tgroup>
      </informaltable>

    </section>

    <section>
    <title>Adding Custom Command-Line Options:  the &AddOption; Function</title>

      <para>

      &SCons; also allows you to define your own
      command-line options with the &AddOption; function.
      The &AddOption; function takes the same arguments
      as the <function>optparse.add_option</function> function
      from the standard Python library.
      <footnote>
      <para>
      The &AddOption; function is,
      in fact, implemented using a subclass
      of the <classname>optparse.OptionParser</classname>.
      </para>
      </footnote>
      Once you have added a custom command-line option
      with the &AddOption; function,
      the value of the option (if any) is immediately available
      using the standard &GetOption; function.
      (The value can also be set using &SetOption;,
      although that's not very useful in practice
      because a default value can be specified in
      directly in the &AddOption; call.)

      </para>

      <para>

      One useful example of using this functionality
      is to provide a <option>--prefix</option> for users:

      </para>

      <programlisting>
        AddOption('--prefix',
                  dest='prefix',
                  type='string',
                  nargs=1,
                  action='store',
                  metavar='DIR',
                  help='installation prefix')

        env = Environment(PREFIX = GetOption('prefix'))

        installed_foo = env.Install('$PREFIX/usr/bin', 'foo.in')
        Default(installed_foo)
      </programlisting>

      <para>

      The above code uses the &GetOption; function
      to set the <varname>$PREFIX</varname>
      construction variable to any
      value that the user specifies with a command-line
      option of <literal>--prefix</literal>.
      Because <varname>$PREFIX</varname>
      will expand to a null string if it's not initialized,
      running &SCons; without the
      option of <literal>--prefix</literal>
      will install the file in the
      <filename>/usr/bin/</filename> directory:

      </para>

      <screen>
        % <userinput>scons -Q -n</userinput>
        Install file: "foo.in" as "/usr/bin/foo.in"
      </screen>

      <para>

      But specifying <literal>--prefix=/tmp/install</literal>
      on the command line causes the file to be installed in the
      <filename>/tmp/install/usr/bin/</filename> directory:

      </para>

      <screen>
        % <userinput>scons -Q -n --prefix=/tmp/install</userinput>
        Install file: "foo.in" as "/tmp/install/usr/bin/foo.in"
      </screen>

    </section>

  </section>

  <section id="sect-command-line-variables">
  <title>Command-Line <varname>variable</varname>=<varname>value</varname> Build Variables</title>

    <para>

    You may want to control various aspects
    of your build by allowing the user
    to specify <varname>variable</varname>=<varname>value</varname>
    values on the command line.
    For example, suppose you
    want users to be able to
    build a debug version of a program
    by running &SCons; as follows:

    </para>

    <screen>
      % <userinput>scons -Q debug=1</userinput>
    </screen>

    <para>

    &SCons; provides an &ARGUMENTS; dictionary
    that stores all of the
    <varname>variable</varname>=<varname>value</varname>
    assignments from the command line.
    This allows you to modify
    aspects of your build in response
    to specifications on the command line.
    (Note that unless you want to require
    that users <emphasis>always</emphasis>
    specify a variable,
    you probably want to use
    the Python
    <literal>ARGUMENTS.get()</literal> function,
    which allows you to specify a default value
    to be used if there is no specification
    on the command line.)

    </para>

    <para>

    The following code sets the &cv-link-CCFLAGS; construction
    variable in response to the <varname>debug</varname>
    flag being set in the &ARGUMENTS; dictionary:

    </para>

    <programlisting>
       env = Environment()
       debug = ARGUMENTS.get('debug', 0)
       if int(debug):
           env.Append(CCFLAGS = '-g')
       env.Program('prog.c')
    </programlisting>

    <para>

    This results in the <varname>-g</varname>
    compiler option being used when
    <literal>debug=1</literal>
    is used on the command line:

    </para>

    <screen>
       % <userinput>scons -Q debug=0</userinput>
       cc -o prog.o -c prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=0</userinput>
       scons: `.' is up to date.
       % <userinput>scons -Q debug=1</userinput>
       cc -o prog.o -c -g prog.c
       cc -o prog prog.o
       % <userinput>scons -Q debug=1</userinput>
       scons: `.' is up to date.
    </screen>

    <para>

    Notice that &SCons; keeps track of
    the last values used to build the object files,
    and as a result correctly rebuilds
    the object and executable files
    only when the value of the <literal>debug</literal>
    argument has changed.

    </para>

    <para>

    The &ARGUMENTS; dictionary has two minor drawbacks.
    First, because it is a dictionary,
    it can only store one value for each specified keyword,
    and thus only "remembers" the last setting
    for each keyword on the command line.
    This makes the &ARGUMENTS; dictionary
    inappropriate if users should be able to
    specify multiple values
    on the command line for a given keyword.
    Second, it does not preserve
    the order in which the variable settings
    were specified,
    which is a problem if
    you want the configuration to
    behave differently in response
    to the order in which the build
    variable settings were specified on the command line.

    </para>

    <para>

    To accomodate these requirements,
    &SCons; provides an &ARGLIST; variable
    that gives you direct access to
    <varname>variable</varname>=<varname>value</varname>
    settings on the command line,
    in the exact order they were specified,
    and without removing any duplicate settings.
    Each element in the &ARGLIST; variable
    is itself a two-element list
    containing the keyword and the value
    of the setting,
    and you must loop through,
    or otherwise select from,
    the elements of &ARGLIST; to
    process the specific settings you want
    in whatever way is appropriate for your configuration.
    For example,
    the following code to let the user
    add to the &CPPDEFINES; construction variable
    by specifying multiple
    <varname>define=</varname>
    settings on the command line:

    </para>

    <programlisting>
       cppdefines = []
       for key, value in ARGLIST:
           if key == 'define':
               cppdefines.append(value)
       env = Environment(CPPDEFINES = cppdefines)
       env.Object('prog.c')
    </programlisting>

    <para>

    Yields the following output:

    </para>

    <screen>
       % <userinput>scons -Q define=FOO</userinput>
       cc -o prog.o -c -DFOO prog.c
       % <userinput>scons -Q define=FOO define=BAR</userinput>
       cc -o prog.o -c -DFOO -DBAR prog.c
    </screen>

    <para>

    Note that the &ARGLIST; and &ARGUMENTS;
    variables do not interfere with each other,
    but merely provide slightly different views
    into how the user specified
    <varname>variable</varname>=<varname>value</varname>
    settings on the command line.
    You can use both variables in the same
    &SCons; configuration.
    In general, the &ARGUMENTS; dictionary
    is more convenient to use,
    (since you can just fetch variable
    settings through a dictionary access),
    and the &ARGLIST; list
    is more flexible
    (since you can examine the
    specific order in which
    the user's command-line variabe settings).

    </para>

    <section>
    <title>Controlling Command-Line Build Variables</title>

      <para>

      Being able to use a command-line build variable like
      <literal>debug=1</literal> is handy,
      but it can be a chore to write specific Python code
      to recognize each such variable,
      check for errors and provide appropriate messages,
      and apply the values to a construction variable.
      To help with this,
      &SCons; supports a class to
      define such build variables easily,
      and a mechanism to apply the
      build variables to a construction environment.
      This allows you to control how the build variables affect
      construction environments.

      </para>

      <para>

      For example, suppose that you want users to set
      a &RELEASE; construction variable on the
      command line whenever the time comes to build
      a program for release,
      and that the value of this variable
      should be added to the command line
      with the appropriate <literal>-D</literal> option
      (or other command line option)
      to pass the value to the C compiler.
      Here's how you might do that by setting
      the appropriate value in a dictionary for the
      &cv-link-CPPDEFINES; construction variable:

      </para>

      <programlisting>
           vars = Variables()
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           env.Program(['foo.c', 'bar.c'])
      </programlisting>

      <para>

      This &SConstruct; file first creates a &Variables; object
      (the <literal>vars = Variables()</literal> call),
      and then uses the object's &Add;
      method to indicate that the &RELEASE;
      variable can be set on the command line,
      and that its default value will be <literal>0</literal>
      (the third argument to the &Add; method).
      The second argument is a line of help text;
      we'll learn how to use it in the next section.

      </para>

      <para>

      We then pass the created &Variables;
      object as a &variables; keyword argument
      to the &Environment; call
      used to create the construction environment.
      This then allows a user to set the
      &RELEASE; build variable on the command line
      and have the variable show up in
      the command line used to build each object from
      a C source file:

      </para>

      <screen>
        % <userinput>scons -Q RELEASE=1</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
        cc -o foo foo.o bar.o
      </screen>

      <para>

      NOTE:  Before &SCons; release 0.98.1, these build variables
      were known as "command-line build options."
      The class was actually named the &Options; class,
      and in the sections below,
      the various functions were named
      &BoolOption;, &EnumOption;, &ListOption;,
      &PathOption;, &PackageOption; and &AddOptions;.
      These older names still work,
      and you may encounter them in older
      &SConscript; fles,
      but they have been officially deprecated
      as of &SCons; version 2.0.

      </para>

    </section>

    <section>
    <title>Providing Help for Command-Line Build Variables</title>

      <para>

      To make command-line build variables most useful,
      you ideally want to provide
      some help text that will describe
      the available variables
      when the user runs <literal>scons -h</literal>.
      You could write this text by hand,
      but &SCons; provides an easier way.
      &Variables; objects support a
      &GenerateHelpText; method
      that will, as its name suggests,
      generate text that describes
      the various variables that
      have been added to it.
      You then pass the output from this method to
      the &Help; function:

      </para>

      <programlisting>
           vars = Variables('custom.py')
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars)
           Help(vars.GenerateHelpText(env))
      </programlisting>

      <para>

      &SCons; will now display some useful text
      when the <literal>-h</literal> option is used:

      </para>

      <screen>
        % <userinput>scons -Q -h</userinput>
        
        RELEASE: Set to 1 to build for release
            default: 0
            actual: 0
        
        Use scons -H for help about command-line options.
      </screen>

      <para>

      Notice that the help output shows the default value,
      and the current actual value of the build variable.

      </para>

    </section>

    <section>
    <title>Reading Build Variables From a File</title>

      <para>

      Giving the user a way to specify the
      value of a build variable on the command line
      is useful,
      but can still be tedious
      if users must specify the variable
      every time they run &SCons;.
      We can let users provide customized build variable settings
      in a local file by providing a
      file name when we create the
      &Variables; object:

      </para>

      <programlisting>
           vars = Variables('custom.py')
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           env.Program(['foo.c', 'bar.c'])
           Help(vars.GenerateHelpText(env))
      </programlisting>

      <para>

      This then allows the user to control the &RELEASE;
      variable by setting it in the &custom_py; file:

      </para>

      <programlisting>
        RELEASE = 1
        </programlisting>

      <para>

      Note that this file is actually executed
      like a Python script.
      Now when we run &SCons;:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=1 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=1 foo.c
        cc -o foo foo.o bar.o
      </screen>

      <para>

      And if we change the contents of &custom_py; to:

      </para>

      <programlisting>
        RELEASE = 0
      </programlisting>

      <para>

      The object files are rebuilt appropriately
      with the new variable:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o bar.o -c -DRELEASE_BUILD=0 bar.c
        cc -o foo.o -c -DRELEASE_BUILD=0 foo.c
        cc -o foo foo.o bar.o
      </screen>

    </section>

    <section>
    <title>Pre-Defined Build Variable Functions</title>

      <para>

      &SCons; provides a number of functions
      that provide ready-made behaviors
      for various types of command-line build variables.

      </para>

      <section>
      <title>True/False Values:  the &BoolVariable; Build Variable Function</title>

        <para>

        It's often handy to be able to specify a
        variable that controls a simple Boolean variable
        with a &true; or &false; value.
        It would be even more handy to accomodate
        users who have different preferences for how to represent
        &true; or &false; values.
        The &BoolVariable; function
        makes it easy to accomodate these
        common representations of
        &true; or &false;.

        </para>

        <para>

        The &BoolVariable; function takes three arguments:
        the name of the build variable,
        the default value of the build variable,
        and the help string for the variable.
        It then returns appropriate information for
        passing to the &Add; method of a &Variables; object, like so:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(BoolVariable('RELEASE', 'Set to build for release', 0))
             env = Environment(variables = vars,
                               CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
             env.Program('foo.c')
        </programlisting>

        <para>

        With this build variable,
        the &RELEASE; variable can now be enabled by
        setting it to the value <literal>yes</literal>
        or <literal>t</literal>:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=yes foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=True foo.c
        </screen>

        <screen>
          % <userinput>scons -Q RELEASE=t foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=True foo.c
        </screen>

        <para>

        Other values that equate to &true; include
        <literal>y</literal>,
        <literal>1</literal>,
        <literal>on</literal>
        and
        <literal>all</literal>.

        </para>

        <para>

        Conversely, &RELEASE; may now be given a &false;
        value by setting it to
        <literal>no</literal>
        or
        <literal>f</literal>:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=no foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=False foo.c
        </screen>

        <screen>
          % <userinput>scons -Q RELEASE=f foo.o</userinput>
          cc -o foo.o -c -DRELEASE_BUILD=False foo.c
        </screen>

        <para>

        Other values that equate to &false; include
        <literal>n</literal>,
        <literal>0</literal>,
        <literal>off</literal>
        and
        <literal>none</literal>.

        </para>

        <para>

        Lastly, if a user tries to specify
        any other value,
        &SCons; supplies an appropriate error message:

        </para>

        <screen>
          % <userinput>scons -Q RELEASE=bad_value foo.o</userinput>
          
          scons: *** Error converting option: RELEASE
          Invalid value for boolean option: bad_value
          File "/home/my/project/SConstruct", line 4, in &lt;module&gt;
        </screen>

      </section>

      <section>
      <title>Single Value From a List:  the &EnumVariable; Build Variable Function</title>

        <para>

        Suppose that we want a user to be able to
        set a &COLOR; variable
        that selects a background color to be
        displayed by an application,
        but that we want to restrict the
        choices to a specific set of allowed colors.
        This can be set up quite easily
        using the &EnumVariable;,
        which takes a list of &allowed_values;
        in addition to the variable name,
        default value,
        and help text arguments:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue')))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        The user can now explicity set the &COLOR; build variable
        to any of the specified allowed values:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="red" foo.c
          % <userinput>scons -Q COLOR=blue foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=green foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

        <para>

        But, almost more importantly,
        an attempt to set &COLOR;
        to a value that's not in the list
        generates an error message:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=magenta foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: magenta
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

        <para>

        The &EnumVariable; function also supports a way
        to map alternate names to allowed values.
        Suppose, for example,
        that we want to allow the user
        to use the word <literal>navy</literal> as a synonym for
        <literal>blue</literal>.
        We do this by adding a &map; dictionary
        that will map its key values
        to the desired legal value:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'}))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        As desired, the user can then use
        <literal>navy</literal> on the command line,
        and &SCons; will translate it into <literal>blue</literal>
        when it comes time to use the &COLOR;
        variable to build a target:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=navy foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
        </screen>

        <para>

        By default, when using the &EnumVariable; function,
        arguments that differ
        from the legal values
        only in case
        are treated as illegal values:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: Red
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
          % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: BLUE
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          
          scons: *** Invalid value for option COLOR: nAvY
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

        <para>

        The &EnumVariable; function can take an additional
        &ignorecase; keyword argument that,
        when set to <literal>1</literal>,
        tells &SCons; to allow case differences
        when the values are specified:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'},
                                 ignorecase=1))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Which yields the output:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="Red" foo.c
          % <userinput>scons -Q COLOR=BLUE foo.o</userinput>
          cc -o foo.o -c -DCOLOR="BLUE" foo.c
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=green foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

        <para>

        Notice that an &ignorecase; value of <literal>1</literal>
        preserves the case-spelling that the user supplied.
        If you want &SCons; to translate the names
        into lower-case,
        regardless of the case used by the user,
        specify an &ignorecase; value of <literal>2</literal>:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(EnumVariable('COLOR', 'Set background color', 'red',
                                 allowed_values=('red', 'green', 'blue'),
                                 map={'navy':'blue'},
                                 ignorecase=2))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLOR' : '"${COLOR}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Now &SCons; will use values of
        <literal>red</literal>,
        <literal>green</literal> or
        <literal>blue</literal>
        regardless of how the user spells
        those values on the command line:

        </para>

        <screen>
          % <userinput>scons -Q COLOR=Red foo.o</userinput>
          cc -o foo.o -c -DCOLOR="red" foo.c
          % <userinput>scons -Q COLOR=nAvY foo.o</userinput>
          cc -o foo.o -c -DCOLOR="blue" foo.c
          % <userinput>scons -Q COLOR=GREEN foo.o</userinput>
          cc -o foo.o -c -DCOLOR="green" foo.c
        </screen>

      </section>

      <section>
      <title>Multiple Values From a List:  the &ListVariable; Build Variable Function</title>

        <para>

        Another way in which you might want to allow users
        to control a build variable is to
        specify a list of one or more legal values.
        &SCons; supports this through the &ListVariable; function.
        If, for example, we want a user to be able to set a
        &COLORS; variable to one or more of the legal list of values:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(ListVariable('COLORS', 'List of colors', 0,
                                 ['red', 'green', 'blue']))
             env = Environment(variables = vars,
                               CPPDEFINES={'COLORS' : '"${COLORS}"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        A user can now specify a comma-separated list
        of legal values,
        which will get translated into a space-separated
        list for passing to the any build commands:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=red,blue foo.o</userinput>
          cc -o foo.o -c -DCOLORS="red blue" foo.c
          % <userinput>scons -Q COLORS=blue,green,red foo.o</userinput>
          cc -o foo.o -c -DCOLORS="blue green red" foo.c
        </screen>

        <para>

        In addition, the &ListVariable; function
        allows the user to specify explicit keywords of
        &all; or &none;
        to select all of the legal values,
        or none of them, respectively:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=all foo.o</userinput>
          cc -o foo.o -c -DCOLORS="red green blue" foo.c
          % <userinput>scons -Q COLORS=none foo.o</userinput>
          cc -o foo.o -c -DCOLORS="" foo.c
        </screen>

        <para>

        And, of course, an illegal value
        still generates an error message:

        </para>

        <screen>
          % <userinput>scons -Q COLORS=magenta foo.o</userinput>
          
          scons: *** Error converting option: COLORS
          Invalid value(s) for option: magenta
          File "/home/my/project/SConstruct", line 5, in &lt;module&gt;
        </screen>

      </section>

      <section>
      <title>Path Names:  the &PathVariable; Build Variable Function</title>

        <para>

        &SCons; supports a &PathVariable; function
        to make it easy to create a build variable
        to control an expected path name.
        If, for example, you need to
        define a variable in the preprocessor
        that controls the location of a
        configuration file:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('CONFIG',
                                 'Path to configuration file',
                                 '/etc/my_config'))
             env = Environment(variables = vars,
                               CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        This then allows the user to
        override the &CONFIG; build variable
        on the command line as necessary:

        </para>

        <screen>
          % <userinput>scons -Q foo.o</userinput>
          cc -o foo.o -c -DCONFIG_FILE="/etc/my_config" foo.c
          % <userinput>scons -Q CONFIG=/usr/local/etc/other_config foo.o</userinput>
          scons: `foo.o' is up to date.
        </screen>

        <para>

        By default, &PathVariable; checks to make sure
        that the specified path exists and generates an error if it
        doesn't:

        </para>

        <screen>
          % <userinput>scons -Q CONFIG=/does/not/exist foo.o</userinput>
          
          scons: *** Path for option CONFIG does not exist: /does/not/exist
          File "/home/my/project/SConstruct", line 6, in &lt;module&gt;
        </screen>

        <para>

        &PathVariable; provides a number of methods
        that you can use to change this behavior.
        If you want to ensure that any specified paths are,
        in fact, files and not directories,
        use the &PathVariable_PathIsFile; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('CONFIG',
                                 'Path to configuration file',
                                 '/etc/my_config',
                                 PathVariable.PathIsFile))
             env = Environment(variables = vars,
                               CPPDEFINES={'CONFIG_FILE' : '"$CONFIG"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Conversely, to ensure that any specified paths are
        directories and not files,
        use the &PathVariable_PathIsDir; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('DBDIR',
                                 'Path to database directory',
                                 '/var/my_dbdir',
                                 PathVariable.PathIsDir))
             env = Environment(variables = vars,
                               CPPDEFINES={'DBDIR' : '"$DBDIR"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        If you want to make sure that any specified paths
        are directories,
        and you would like the directory created
        if it doesn't already exist,
        use the &PathVariable_PathIsDirCreate; method:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('DBDIR',
                                 'Path to database directory',
                                 '/var/my_dbdir',
                                 PathVariable.PathIsDirCreate))
             env = Environment(variables = vars,
                               CPPDEFINES={'DBDIR' : '"$DBDIR"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        Lastly, if you don't care whether the path exists,
        is a file, or a directory,
        use the &PathVariable_PathAccept; method
        to accept any path that the user supplies:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PathVariable('OUTPUT',
                                 'Path to output file or directory',
                                 None,
                                 PathVariable.PathAccept))
             env = Environment(variables = vars,
                               CPPDEFINES={'OUTPUT' : '"$OUTPUT"'})
             env.Program('foo.c')
        </programlisting>

      </section>

      <section>
      <title>Enabled/Disabled Path Names: the &PackageVariable; Build Variable Function</title>

        <para>

        Sometimes you want to give users
        even more control over a path name variable,
        allowing them to explicitly enable or
        disable the path name
        by using <literal>yes</literal> or <literal>no</literal> keywords,
        in addition to allow them
        to supply an explicit path name.
        &SCons; supports the &PackageVariable;
        function to support this:

        </para>

        <programlisting>
             vars = Variables('custom.py')
             vars.Add(PackageVariable('PACKAGE',
                                    'Location package',
                                    '/opt/location'))
             env = Environment(variables = vars,
                               CPPDEFINES={'PACKAGE' : '"$PACKAGE"'})
             env.Program('foo.c')
        </programlisting>

        <para>

        When the &SConscript; file uses the &PackageVariable; funciton,
        user can now still use the default
        or supply an overriding path name,
        but can now explicitly set the
        specified variable to a value
        that indicates the package should be enabled
        (in which case the default should be used)
        or disabled:

        </para>

        <screen>
          % <userinput>scons -Q foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="/opt/location" foo.c
          % <userinput>scons -Q PACKAGE=/usr/local/location foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="/usr/local/location" foo.c
          % <userinput>scons -Q PACKAGE=yes foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="True" foo.c
          % <userinput>scons -Q PACKAGE=no foo.o</userinput>
          cc -o foo.o -c -DPACKAGE="False" foo.c
        </screen>

      </section>

    </section>

    <section>
    <title>Adding Multiple Command-Line Build Variables at Once</title>

      <para>

      Lastly, &SCons; provides a way to add
      multiple build variables to a &Variables; object at once.
      Instead of having to call the &Add; method
      multiple times,
      you can call the &AddVariables;
      method with a list of build variables
      to be added to the object.
      Each build variable is specified
      as either a tuple of arguments,
      just like you'd pass to the &Add; method itself,
      or as a call to one of the pre-defined
      functions for pre-packaged command-line build variables.
      in any order:

      </para>

      <programlisting>
          vars = Variables()
          vars.AddVariables(
              ('RELEASE', 'Set to 1 to build for release', 0),
              ('CONFIG', 'Configuration file', '/etc/my_config'),
              BoolVariable('warnings', 'compilation with -Wall and similiar', 1),
              EnumVariable('debug', 'debug output and symbols', 'no',
                         allowed_values=('yes', 'no', 'full'),
                         map={}, ignorecase=0),  # case sensitive
              ListVariable('shared',
                         'libraries to build as shared libraries',
                         'all',
                         names = list_of_libs),
              PackageVariable('x11',
                            'use X11 installed here (yes = search some places)',
                            'yes'),
              PathVariable('qtdir', 'where the root of Qt is installed', qtdir),
          )
      </programlisting>

      <para>
      </para>

    </section>

    <section>
    <title>Handling Unknown Command-Line Build Variables:  the &UnknownVariables; Function</title>

      <para>

      Users may, of course,
      occasionally misspell variable names in their command-line settings.
      &SCons; does not generate an error or warning
      for any unknown variables the users specifies on the command line.
      (This is in no small part because you may be
      processing the arguments directly using the &ARGUMENTS; dictionary,
      and therefore &SCons; can't know in the general case
      whether a given "misspelled" variable is
      really unknown and a potential problem,
      or something that your &SConscript; file
      will handle directly with some Python code.)

      </para>

      <para>

      If, however, you're using a &Variables; object to
      define a specific set of command-line build variables
      that you expect users to be able to set,
      you may want to provide an error
      message or warning of your own
      if the user supplies a variable setting
      that is <emphasis>not</emphasis> among
      the defined list of variable names known to the &Variables; object.
      You can do this by calling the &UnknownVariables;
      method of the &Variables; object:

      </para>

      <programlisting>
           vars = Variables(None)
           vars.Add('RELEASE', 'Set to 1 to build for release', 0)
           env = Environment(variables = vars,
                             CPPDEFINES={'RELEASE_BUILD' : '${RELEASE}'})
           unknown = vars.UnknownVariables()
           if unknown:
               print "Unknown variables:", unknown.keys()
               Exit(1)
           env.Program('foo.c')
      </programlisting>

      <para>

      The &UnknownVariables; method returns a dictionary
      containing the keywords and values
      of any variables the user specified on the command line
      that are <emphasis>not</emphasis>
      among the variables known to the &Variables; object
      (from having been specified using
      the &Variables; object's&Add; method).
      In the examble above,
      we check for whether the dictionary
      returned by the &UnknownVariables; is non-empty,
      and if so print the Python list
      containing the names of the unknwown variables
      and then call the &Exit; function
      to terminate &SCons;:

      </para>

      <screen>
        % <userinput>scons -Q NOT_KNOWN=foo</userinput>
        Unknown variables: ['NOT_KNOWN']
      </screen>

      <para>

      Of course, you can process the items in the
      dictionary returned by the &UnknownVariables; function
      in any way appropriate to your build configuration,
      including just printing a warning message
      but not exiting,
      logging an error somewhere,
      etc.

      </para>

      <para>

      Note that you must delay the call of &UnknownVariables;
      until after you have applied the &Variables; object
      to a construction environment
      with the <literal>variables=</literal>
      keyword argument of an &Environment; call.

      </para>

    </section>

  </section>

  <section id="sect-command-line-targets">
  <title>Command-Line Targets</title>

    <section>
    <title>Fetching Command-Line Targets: the &COMMAND_LINE_TARGETS; Variable</title>

      <para>

      &SCons; supports a &COMMAND_LINE_TARGETS; variable
      that lets you fetch the list of targets that the
      user specified on the command line.
      You can use the targets to manipulate the
      build in any way you wish.
      As a simple example,
      suppose that you want to print a reminder
      to the user whenever a specific program is built.
      You can do this by checking for the
      target in the &COMMAND_LINE_TARGETS; list:

      </para>

      <programlisting>
        if 'bar' in COMMAND_LINE_TARGETS:
            print "Don't forget to copy `bar' to the archive!"
        Default(Program('foo.c'))
        Program('bar.c')
      </programlisting>

      <para>

      Then, running &SCons; with the default target
      works as it always does,
      but explicity specifying the &bar; target
      on the command line generates the warning message:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o foo.o -c foo.c
        cc -o foo foo.o
        % <userinput>scons -Q bar</userinput>
        Don't forget to copy `bar' to the archive!
        cc -o bar.o -c bar.c
        cc -o bar bar.o
      </screen>

      <para>

      Another practical use for the &COMMAND_LINE_TARGETS; variable
      might be to speed up a build
      by only reading certain subsidiary &SConscript;
      files if a specific target is requested.

      </para>

    </section>

    <section>
    <title>Controlling the Default Targets:  the &Default; Function</title>

      <para>

      One of the most basic things you can control
      is which targets &SCons; will build by default--that is,
      when there are no targets specified on the command line.
      As mentioned previously,
      &SCons; will normally build every target
      in or below the current directory
      by default--that is, when you don't
      explicitly specify one or more targets
      on the command line.
      Sometimes, however, you may want
      to specify explicitly that only
      certain programs, or programs in certain directories,
      should be built by default.
      You do this with the &Default; function:

      </para>

      <programlisting>
         env = Environment()
         hello = env.Program('hello.c')
         env.Program('goodbye.c')
         Default(hello)
      </programlisting>

      <para>

      This &SConstruct; file knows how to build two programs,
      &hello; and &goodbye;,
      but only builds the
      &hello; program by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o hello.o -c hello.c
         cc -o hello hello.o
         % <userinput>scons -Q</userinput>
         scons: `hello' is up to date.
         % <userinput>scons -Q goodbye</userinput>
         cc -o goodbye.o -c goodbye.c
         cc -o goodbye goodbye.o
      </screen>

      <para>

      Note that, even when you use the &Default;
      function in your &SConstruct; file,
      you can still explicitly specify the current directory
      (<literal>.</literal>) on the command line
      to tell &SCons; to build
      everything in (or below) the current directory:

      </para>

      <screen>
         % <userinput>scons -Q .</userinput>
         cc -o goodbye.o -c goodbye.c
         cc -o goodbye goodbye.o
         cc -o hello.o -c hello.c
         cc -o hello hello.o
      </screen>

      <para>

      You can also call the &Default;
      function more than once,
      in which case each call
      adds to the list of targets to be built by default:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         Default(prog1)
         prog2 = env.Program('prog2.c')
         prog3 = env.Program('prog3.c')
         Default(prog3)
      </programlisting>

      <para>

      Or you can specify more than one target
      in a single call to the &Default; function:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         prog2 = env.Program('prog2.c')
         prog3 = env.Program('prog3.c')
         Default(prog1, prog3)
      </programlisting>

      <para>

      Either of these last two examples
      will build only the
      <application>prog1</application>
      and
      <application>prog3</application>
      programs by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o prog1.o -c prog1.c
         cc -o prog1 prog1.o
         cc -o prog3.o -c prog3.c
         cc -o prog3 prog3.o
         % <userinput>scons -Q .</userinput>
         cc -o prog2.o -c prog2.c
         cc -o prog2 prog2.o
      </screen>

      <para>

      You can list a directory as
      an argument to &Default;:

      </para>

      <programlisting>
         env = Environment()
         env.Program(['prog1/main.c', 'prog1/foo.c'])
         env.Program(['prog2/main.c', 'prog2/bar.c'])
         Default('prog1')
      </programlisting>

      <para>

      In which case only the target(s) in that
      directory will be built by default:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         cc -o prog1/foo.o -c prog1/foo.c
         cc -o prog1/main.o -c prog1/main.c
         cc -o prog1/main prog1/main.o prog1/foo.o
         % <userinput>scons -Q</userinput>
         scons: `prog1' is up to date.
         % <userinput>scons -Q .</userinput>
         cc -o prog2/bar.o -c prog2/bar.c
         cc -o prog2/main.o -c prog2/main.c
         cc -o prog2/main prog2/main.o prog2/bar.o
      </screen>

      <para>

      Lastly, if for some reason you don't want
      any targets built by default,
      you can use the Python <literal>None</literal>
      variable:

      </para>

      <programlisting>
         env = Environment()
         prog1 = env.Program('prog1.c')
         prog2 = env.Program('prog2.c')
         Default(None)
      </programlisting>

      <para>

      Which would produce build output like:

      </para>

      <screen>
         % <userinput>scons -Q</userinput>
         scons: *** No targets specified and no Default() targets found.  Stop.
         % <userinput>scons -Q .</userinput>
         cc -o prog1.o -c prog1.c
         cc -o prog1 prog1.o
         cc -o prog2.o -c prog2.c
         cc -o prog2 prog2.o
      </screen>

      <section>
      <title>Fetching the List of Default Targets: the &DEFAULT_TARGETS; Variable</title>

        <para>

        &SCons; supports a &DEFAULT_TARGETS; variable
        that lets you get at the current list of default targets.
        The &DEFAULT_TARGETS; variable has
        two important differences from the &COMMAND_LINE_TARGETS; variable.
        First, the &DEFAULT_TARGETS; variable is a list of
        internal &SCons; nodes,
        so you need to convert the list elements to strings
        if you want to print them or look for a specific target name.
        Fortunately, you can do this easily
        by using the Python <function>map</function> function
        to run the list through <function>str</function>:

        </para>

        <programlisting>
           prog1 = Program('prog1.c')
           Default(prog1)
           print "DEFAULT_TARGETS is", map(str, DEFAULT_TARGETS)
        </programlisting>

        <para>

        (Keep in mind that all of the manipulation of the
        &DEFAULT_TARGETS; list takes place during the
        first phase when &SCons; is reading up the &SConscript; files,
        which is obvious if
        we leave off the <literal>-Q</literal> flag when we run &SCons;:)

        </para>

        <screen>
           % <userinput>scons</userinput>
           scons: Reading SConscript files ...
           DEFAULT_TARGETS is ['prog1']
           scons: done reading SConscript files.
           scons: Building targets ...
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           scons: done building targets.
        </screen>

        <para>

        Second,
        the contents of the &DEFAULT_TARGETS; list change
        in response to calls to the &Default;: function,
        as you can see from the following &SConstruct; file:

        </para>

        <programlisting>
           prog1 = Program('prog1.c')
           Default(prog1)
           print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
           prog2 = Program('prog2.c')
           Default(prog2)
           print "DEFAULT_TARGETS is now", map(str, DEFAULT_TARGETS)
        </programlisting>

        <para>

        Which yields the output:

        </para>

        <screen>
           % <userinput>scons</userinput>
           scons: Reading SConscript files ...
           DEFAULT_TARGETS is now ['prog1']
           DEFAULT_TARGETS is now ['prog1', 'prog2']
           scons: done reading SConscript files.
           scons: Building targets ...
           cc -o prog1.o -c prog1.c
           cc -o prog1 prog1.o
           cc -o prog2.o -c prog2.c
           cc -o prog2 prog2.o
           scons: done building targets.
        </screen>

        <para>

        In practice, this simply means that you
        need to pay attention to the order in
        which you call the &Default; function
        and refer to the &DEFAULT_TARGETS; list,
        to make sure that you don't examine the
        list before you've added the default targets
        you expect to find in it.

        </para>

      </section>

    </section>

    <section>
    <title>Fetching the List of Build Targets, Regardless of Origin: the &BUILD_TARGETS; Variable</title>

      <para>

      We've already been introduced to the
      &COMMAND_LINE_TARGETS; variable,
      which contains a list of targets specified on the command line,
      and the &DEFAULT_TARGETS; variable,
      which contains a list of targets specified
      via calls to the &Default; method or function.
      Sometimes, however,
      you want a list of whatever targets
      &SCons; will try to build,
      regardless of whether the targets came from the
      command line or a &Default; call.
      You could code this up by hand, as follows:

      </para>

      <programlisting>
        if COMMAND_LINE_TARGETS:
            targets = COMMAND_LINE_TARGETS
        else:
            targets = DEFAULT_TARGETS
      </programlisting>

      <para>

      &SCons;, however, provides a convenient
      &BUILD_TARGETS; variable
      that eliminates the need for this by-hand manipulation.
      Essentially, the &BUILD_TARGETS; variable
      contains a list of the command-line targets,
      if any were specified,
      and if no command-line targets were specified,
      it contains a list of the targets specified
      via the &Default; method or function.

      </para>

      <para>

      Because &BUILD_TARGETS; may contain a list of &SCons; nodes,
      you must convert the list elements to strings
      if you want to print them or look for a specific target name,
      just like the &DEFAULT_TARGETS; list:

      </para>

      <programlisting>
        prog1 = Program('prog1.c')
        Program('prog2.c')
        Default(prog1)
        print "BUILD_TARGETS is", map(str, BUILD_TARGETS)
      </programlisting>

      <para>

      Notice how the value of &BUILD_TARGETS;
      changes depending on whether a target is
      specified on the command line:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        BUILD_TARGETS is ['prog1']
        cc -o prog1.o -c prog1.c
        cc -o prog1 prog1.o
        % <userinput>scons -Q prog2</userinput>
        BUILD_TARGETS is ['prog2']
        cc -o prog2.o -c prog2.c
        cc -o prog2 prog2.o
        % <userinput>scons -Q -c .</userinput>
        BUILD_TARGETS is ['.']
        Removed prog1.o
        Removed prog1
        Removed prog2.o
        Removed prog2
      </screen>

    </section>

  </section>