aboutsummaryrefslogtreecommitdiffstats
path: root/manual/pattern.texi
blob: 24e8e4c61204436a2ac03c36ad146a900c0028f3 (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
@node Pattern Matching, I/O Overview, Searching and Sorting, Top
@c %MENU% Matching shell ``globs'' and regular expressions
@chapter Pattern Matching

The GNU C Library provides pattern matching facilities for two kinds of
patterns: regular expressions and file-name wildcards.  The library also
provides a facility for expanding variable and command references and
parsing text into words in the way the shell does.

@menu
* Wildcard Matching::    Matching a wildcard pattern against a single string.
* Globbing::             Finding the files that match a wildcard pattern.
* Regular Expressions::  Matching regular expressions against strings.
* Word Expansion::       Expanding shell variables, nested commands,
			    arithmetic, and wildcards.
			    This is what the shell does with shell commands.
@end menu

@node Wildcard Matching
@section Wildcard Matching

@pindex fnmatch.h
This section describes how to match a wildcard pattern against a
particular string.  The result is a yes or no answer: does the
string fit the pattern or not.  The symbols described here are all
declared in @file{fnmatch.h}.

@comment fnmatch.h
@comment POSIX.2
@deftypefun int fnmatch (const char *@var{pattern}, const char *@var{string}, int @var{flags})
This function tests whether the string @var{string} matches the pattern
@var{pattern}.  It returns @code{0} if they do match; otherwise, it
returns the nonzero value @code{FNM_NOMATCH}.  The arguments
@var{pattern} and @var{string} are both strings.

The argument @var{flags} is a combination of flag bits that alter the
details of matching.  See below for a list of the defined flags.

In the GNU C Library, @code{fnmatch} cannot experience an ``error''---it
always returns an answer for whether the match succeeds.  However, other
implementations of @code{fnmatch} might sometimes report ``errors''.
They would do so by returning nonzero values that are not equal to
@code{FNM_NOMATCH}.
@end deftypefun

These are the available flags for the @var{flags} argument:

@table @code
@comment fnmatch.h
@comment GNU
@item FNM_FILE_NAME
Treat the @samp{/} character specially, for matching file names.  If
this flag is set, wildcard constructs in @var{pattern} cannot match
@samp{/} in @var{string}.  Thus, the only way to match @samp{/} is with
an explicit @samp{/} in @var{pattern}.

@comment fnmatch.h
@comment POSIX.2
@item FNM_PATHNAME
This is an alias for @code{FNM_FILE_NAME}; it comes from POSIX.2.  We
don't recommend this name because we don't use the term ``pathname'' for
file names.

@comment fnmatch.h
@comment POSIX.2
@item FNM_PERIOD
Treat the @samp{.} character specially if it appears at the beginning of
@var{string}.  If this flag is set, wildcard constructs in @var{pattern}
cannot match @samp{.} as the first character of @var{string}.

If you set both @code{FNM_PERIOD} and @code{FNM_FILE_NAME}, then the
special treatment applies to @samp{.} following @samp{/} as well as to
@samp{.} at the beginning of @var{string}.  (The shell uses the
@code{FNM_PERIOD} and @code{FNM_FILE_NAME} flags together for matching
file names.)

@comment fnmatch.h
@comment POSIX.2
@item FNM_NOESCAPE
Don't treat the @samp{\} character specially in patterns.  Normally,
@samp{\} quotes the following character, turning off its special meaning
(if any) so that it matches only itself.  When quoting is enabled, the
pattern @samp{\?} matches only the string @samp{?}, because the question
mark in the pattern acts like an ordinary character.

If you use @code{FNM_NOESCAPE}, then @samp{\} is an ordinary character.

@comment fnmatch.h
@comment GNU
@item FNM_LEADING_DIR
Ignore a trailing sequence of characters starting with a @samp{/} in
@var{string}; that is to say, test whether @var{string} starts with a
directory name that @var{pattern} matches.

If this flag is set, either @samp{foo*} or @samp{foobar} as a pattern
would match the string @samp{foobar/frobozz}.

@comment fnmatch.h
@comment GNU
@item FNM_CASEFOLD
Ignore case in comparing @var{string} to @var{pattern}.
@end table

@node Globbing
@section Globbing

@cindex globbing
The archetypal use of wildcards is for matching against the files in a
directory, and making a list of all the matches.  This is called
@dfn{globbing}.

You could do this using @code{fnmatch}, by reading the directory entries
one by one and testing each one with @code{fnmatch}.  But that would be
slow (and complex, since you would have to handle subdirectories by
hand).

The library provides a function @code{glob} to make this particular use
of wildcards convenient.  @code{glob} and the other symbols in this
section are declared in @file{glob.h}.

@menu
* Calling Glob::             Basic use of @code{glob}.
* Flags for Globbing::       Flags that enable various options in @code{glob}.
* More Flags for Globbing::  GNU specific extensions to @code{glob}.
@end menu

@node Calling Glob
@subsection Calling @code{glob}

The result of globbing is a vector of file names (strings).  To return
this vector, @code{glob} uses a special data type, @code{glob_t}, which
is a structure.  You pass @code{glob} the address of the structure, and
it fills in the structure's fields to tell you about the results.

@comment glob.h
@comment POSIX.2
@deftp {Data Type} glob_t
This data type holds a pointer to a word vector.  More precisely, it
records both the address of the word vector and its size.  The GNU
implementation contains some more fields which are non-standard
extensions.

@table @code
@item gl_pathc
The number of elements in the vector.

@item gl_pathv
The address of the vector.  This field has type @w{@code{char **}}.

@item gl_offs
The offset of the first real element of the vector, from its nominal
address in the @code{gl_pathv} field.  Unlike the other fields, this
is always an input to @code{glob}, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty.  (The @code{glob} function fills them with
null pointers.)

The @code{gl_offs} field is meaningful only if you use the
@code{GLOB_DOOFFS} flag.  Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.

@item gl_closedir
The address of an alternative implementation of the @code{closedir}
function.  It is used if the @code{GLOB_ALTDIRFUNC} bit is set in
the flag parameter.  The type of this field is
@w{@code{void (*) (void *)}}.

This is a GNU extension.

@item gl_readdir
The address of an alternative implementation of the @code{readdir}
function used to read the contents of a directory.  It is used if the
@code{GLOB_ALTDIRFUNC} bit is set in the flag parameter.  The type of
this field is @w{@code{struct dirent *(*) (void *)}}.

This is a GNU extension.

@item gl_opendir
The address of an alternative implementation of the @code{opendir}
function.  It is used if the @code{GLOB_ALTDIRFUNC} bit is set in
the flag parameter.  The type of this field is
@w{@code{void *(*) (const char *)}}.

This is a GNU extension.

@item gl_stat
The address of an alternative implementation of the @code{stat} function
to get information about an object in the filesystem.  It is used if the
@code{GLOB_ALTDIRFUNC} bit is set in the flag parameter.  The type of
this field is @w{@code{int (*) (const char *, struct stat *)}}.

This is a GNU extension.

@item gl_lstat
The address of an alternative implementation of the @code{lstat}
function to get information about an object in the filesystems, not
following symbolic links.  It is used if the @code{GLOB_ALTDIRFUNC} bit
is set in the flag parameter.  The type of this field is @code{@w{int
(*) (const char *,} @w{struct stat *)}}.

This is a GNU extension.
@end table
@end deftp

@comment glob.h
@comment POSIX.2
@deftypefun int glob (const char *@var{pattern}, int @var{flags}, int (*@var{errfunc}) (const char *@var{filename}, int @var{error-code}), glob_t *@var{vector-ptr})
The function @code{glob} does globbing using the pattern @var{pattern}
in the current directory.  It puts the result in a newly allocated
vector, and stores the size and address of this vector into
@code{*@var{vector-ptr}}.  The argument @var{flags} is a combination of
bit flags; see @ref{Flags for Globbing}, for details of the flags.

The result of globbing is a sequence of file names.  The function
@code{glob} allocates a string for each resulting word, then
allocates a vector of type @code{char **} to store the addresses of
these strings.  The last element of the vector is a null pointer.
This vector is called the @dfn{word vector}.

To return this vector, @code{glob} stores both its address and its
length (number of elements, not counting the terminating null pointer)
into @code{*@var{vector-ptr}}.

Normally, @code{glob} sorts the file names alphabetically before
returning them.  You can turn this off with the flag @code{GLOB_NOSORT}
if you want to get the information as fast as possible.  Usually it's
a good idea to let @code{glob} sort them---if you process the files in
alphabetical order, the users will have a feel for the rate of progress
that your application is making.

If @code{glob} succeeds, it returns 0.  Otherwise, it returns one
of these error codes:

@table @code
@comment glob.h
@comment POSIX.2
@item GLOB_ABORTED
There was an error opening a directory, and you used the flag
@code{GLOB_ERR} or your specified @var{errfunc} returned a nonzero
value.
@iftex
See below
@end iftex
@ifinfo
@xref{Flags for Globbing},
@end ifinfo
for an explanation of the @code{GLOB_ERR} flag and @var{errfunc}.

@comment glob.h
@comment POSIX.2
@item GLOB_NOMATCH
The pattern didn't match any existing files.  If you use the
@code{GLOB_NOCHECK} flag, then you never get this error code, because
that flag tells @code{glob} to @emph{pretend} that the pattern matched
at least one file.

@comment glob.h
@comment POSIX.2
@item GLOB_NOSPACE
It was impossible to allocate memory to hold the result.
@end table

In the event of an error, @code{glob} stores information in
@code{*@var{vector-ptr}} about all the matches it has found so far.
@end deftypefun

@node Flags for Globbing
@subsection Flags for Globbing

This section describes the flags that you can specify in the
@var{flags} argument to @code{glob}.  Choose the flags you want,
and combine them with the C bitwise OR operator @code{|}.

@table @code
@comment glob.h
@comment POSIX.2
@item GLOB_APPEND
Append the words from this expansion to the vector of words produced by
previous calls to @code{glob}.  This way you can effectively expand
several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the
word vector structure between calls to @code{glob}.  And, if you set
@code{GLOB_DOOFFS} in the first call to @code{glob}, you must also
set it when you append to the results.

Note that the pointer stored in @code{gl_pathv} may no longer be valid
after you call @code{glob} the second time, because @code{glob} might
have relocated the vector.  So always fetch @code{gl_pathv} from the
@code{glob_t} structure after each @code{glob} call; @strong{never} save
the pointer across calls.

@comment glob.h
@comment POSIX.2
@item GLOB_DOOFFS
Leave blank slots at the beginning of the vector of words.
The @code{gl_offs} field says how many slots to leave.
The blank slots contain null pointers.

@comment glob.h
@comment POSIX.2
@item GLOB_ERR
Give up right away and report an error if there is any difficulty
reading the directories that must be read in order to expand @var{pattern}
fully.  Such difficulties might include a directory in which you don't
have the requisite access.  Normally, @code{glob} tries its best to keep
on going despite any errors, reading whatever directories it can.

You can exercise even more control than this by specifying an
error-handler function @var{errfunc} when you call @code{glob}.  If
@var{errfunc} is not a null pointer, then @code{glob} doesn't give up
right away when it can't read a directory; instead, it calls
@var{errfunc} with two arguments, like this:

@smallexample
(*@var{errfunc}) (@var{filename}, @var{error-code})
@end smallexample

@noindent
The argument @var{filename} is the name of the directory that
@code{glob} couldn't open or couldn't read, and @var{error-code} is the
@code{errno} value that was reported to @code{glob}.

If the error handler function returns nonzero, then @code{glob} gives up
right away.  Otherwise, it continues.

@comment glob.h
@comment POSIX.2
@item GLOB_MARK
If the pattern matches the name of a directory, append @samp{/} to the
directory's name when returning it.

@comment glob.h
@comment POSIX.2
@item GLOB_NOCHECK
If the pattern doesn't match any file names, return the pattern itself
as if it were a file name that had been matched.  (Normally, when the
pattern doesn't match anything, @code{glob} returns that there were no
matches.)

@comment glob.h
@comment POSIX.2
@item GLOB_NOSORT
Don't sort the file names; return them in no particular order.
(In practice, the order will depend on the order of the entries in
the directory.)  The only reason @emph{not} to sort is to save time.

@comment glob.h
@comment POSIX.2
@item GLOB_NOESCAPE
Don't treat the @samp{\} character specially in patterns.  Normally,
@samp{\} quotes the following character, turning off its special meaning
(if any) so that it matches only itself.  When quoting is enabled, the
pattern @samp{\?} matches only the string @samp{?}, because the question
mark in the pattern acts like an ordinary character.

If you use @code{GLOB_NOESCAPE}, then @samp{\} is an ordinary character.

@code{glob} does its work by calling the function @code{fnmatch}
repeatedly.  It handles the flag @code{GLOB_NOESCAPE} by turning on the
@code{FNM_NOESCAPE} flag in calls to @code{fnmatch}.
@end table

@node More Flags for Globbing
@subsection More Flags for Globbing

Beside the flags described in the last section, the GNU implementation of
@code{glob} allows a few more flags which are also defined in the
@file{glob.h} file.  Some of the extensions implement functionality
which is available in modern shell implementations.

@table @code
@comment glob.h
@comment GNU
@item GLOB_PERIOD
The @code{.} character (period) is treated special.  It cannot be
matched by wildcards.  @xref{Wildcard Matching}, @code{FNM_PERIOD}.

@comment glob.h
@comment GNU
@item GLOB_MAGCHAR
The @code{GLOB_MAGCHAR} value is not to be given to @code{glob} in the
@var{flags} parameter.  Instead, @code{glob} sets this bit in the
@var{gl_flags} element of the @var{glob_t} structure provided as the
result if the pattern used for matching contains any wildcard character.

@comment glob.h
@comment GNU
@item GLOB_ALTDIRFUNC
Instead of the using the using the normal functions for accessing the
filesystem the @code{glob} implementation uses the user-supplied
functions specified in the structure pointed to by @var{pglob}
parameter.  For more information about the functions refer to the
sections about directory handling @ref{Accessing Directories} and
@ref{Reading Attributes}.

@comment glob.h
@comment GNU
@item GLOB_BRACE
If this flag is given the handling of braces in the pattern is changed.
It is now required that braces appear correctly grouped.  I.e., for each
opening brace there must be a closing one.  Braces can be used
recursively.  So it is possible to define one brace expression in
another one.  It is important to note that the range of each brace
expression is completely contained in the outer brace expression (if
there is one).

The string between the matching braces is separated into single
expressions by splitting at @code{,} (comma) characters.  The commas
themself are discarded.  Please note what we said above about recursive
brace expressions.  The commas used to separate the subexpressions must
be at the same level.  Commas in brace subexpressions are not matched.
They are used during expansion of the brace expression of the deeper
level.  The example below shows this

@smallexample
glob ("@{foo/@{,bar,biz@},baz@}", GLOB_BRACE, NULL, &result)
@end smallexample

@noindent
is equivalent to the sequence

@smallexample
glob ("foo/", GLOB_BRACE, NULL, &result)
glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
@end smallexample

@noindent
if we leave aside error handling.

@comment glob.h
@comment GNU
@item GLOB_NOMAGIC
If the pattern contains no wildcard constructs (it is a literal file name),
return it as the sole ``matching'' word, even if no file exists by that name.

@comment glob.h
@comment GNU
@item GLOB_TILDE
If this flag is used the character @code{~} (tilde) is handled special
if it appears at the beginning of the pattern.  Instead of being taken
verbatim it is used to represent the home directory of a known user.

If @code{~} is the only character in pattern or it is followed by a
@code{/} (slash), the home directory of the process owner is
substituted.  Using @code{getlogin} and @code{getpwnam} the information
is read from the system databases.  As an example take user @code{bart}
with his home directory at @file{/home/bart}.  For him a call like

@smallexample
glob ("~/bin/*", GLOB_TILDE, NULL, &result)
@end smallexample

@noindent
would return the contents of the directory @file{/home/bart/bin}.
Instead of referring to the own home directory it is also possible to
name the home directory of other users.  To do so one has to append the
user name after the tilde character.  So the contents of user
@code{homer}'s @file{bin} directory can be retrieved by

@smallexample
glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)
@end smallexample

This functionality is equivalent to what is available in C-shells.

@comment glob.h
@comment GNU
@item GLOB_ONLYDIR
If this flag is used the globbing function takes this as a
@strong{hint} that the caller is only interested in directories
matching the pattern.  If the information about the type of the file
is easily available non-directories will be rejected but no extra
work will be done to determine the information for each file.  I.e.,
the caller must still be able to filter directories out.

This functionality is only available with the GNU @code{glob}
implementation.  It is mainly used internally to increase the
performance but might be useful for a user as well and therefore is
documented here.
@end table

Calling @code{glob} will in most cases allocate resources which are used
to represent the result of the function call.  If the same object of
type @code{glob_t} is used in multiple call to @code{glob} the resources
are freed or reused so that no leaks appear.  But this does not include
the time when all @code{glob} calls are done.

@comment glob.h
@comment POSIX.2
@deftypefun void globfree (glob_t *@var{pglob})
The @code{globfree} function frees all resources allocated by previous
calls to @code{glob} associated with the object pointed to by
@var{pglob}.  This function should be called whenever the currently used
@code{glob_t} typed object isn't used anymore.
@end deftypefun


@node Regular Expressions
@section Regular Expression Matching

The GNU C library supports two interfaces for matching regular
expressions.  One is the standard POSIX.2 interface, and the other is
what the GNU system has had for many years.

Both interfaces are declared in the header file @file{regex.h}.
If you define @w{@code{_POSIX_C_SOURCE}}, then only the POSIX.2
functions, structures, and constants are declared.
@c !!! we only document the POSIX.2 interface here!!

@menu
* POSIX Regexp Compilation::    Using @code{regcomp} to prepare to match.
* Flags for POSIX Regexps::     Syntax variations for @code{regcomp}.
* Matching POSIX Regexps::      Using @code{regexec} to match the compiled
				   pattern that you get from @code{regcomp}.
* Regexp Subexpressions::       Finding which parts of the string were matched.
* Subexpression Complications:: Find points of which parts were matched.
* Regexp Cleanup::		Freeing storage; reporting errors.
@end menu

@node POSIX Regexp Compilation
@subsection POSIX Regular Expression Compilation

Before you can actually match a regular expression, you must
@dfn{compile} it.  This is not true compilation---it produces a special
data structure, not machine instructions.  But it is like ordinary
compilation in that its purpose is to enable you to ``execute'' the
pattern fast.  (@xref{Matching POSIX Regexps}, for how to use the
compiled regular expression for matching.)

There is a special data type for compiled regular expressions:

@comment regex.h
@comment POSIX.2
@deftp {Data Type} regex_t
This type of object holds a compiled regular expression.
It is actually a structure.  It has just one field that your programs
should look at:

@table @code
@item re_nsub
This field holds the number of parenthetical subexpressions in the
regular expression that was compiled.
@end table

There are several other fields, but we don't describe them here, because
only the functions in the library should use them.
@end deftp

After you create a @code{regex_t} object, you can compile a regular
expression into it by calling @code{regcomp}.

@comment regex.h
@comment POSIX.2
@deftypefun int regcomp (regex_t *@var{compiled}, const char *@var{pattern}, int @var{cflags})
The function @code{regcomp} ``compiles'' a regular expression into a
data structure that you can use with @code{regexec} to match against a
string.  The compiled regular expression format is designed for
efficient matching.  @code{regcomp} stores it into @code{*@var{compiled}}.

It's up to you to allocate an object of type @code{regex_t} and pass its
address to @code{regcomp}.

The argument @var{cflags} lets you specify various options that control
the syntax and semantics of regular expressions.  @xref{Flags for POSIX
Regexps}.

If you use the flag @code{REG_NOSUB}, then @code{regcomp} omits from
the compiled regular expression the information necessary to record
how subexpressions actually match.  In this case, you might as well
pass @code{0} for the @var{matchptr} and @var{nmatch} arguments when
you call @code{regexec}.

If you don't use @code{REG_NOSUB}, then the compiled regular expression
does have the capacity to record how subexpressions match.  Also,
@code{regcomp} tells you how many subexpressions @var{pattern} has, by
storing the number in @code{@var{compiled}->re_nsub}.  You can use that
value to decide how long an array to allocate to hold information about
subexpression matches.

@code{regcomp} returns @code{0} if it succeeds in compiling the regular
expression; otherwise, it returns a nonzero error code (see the table
below).  You can use @code{regerror} to produce an error message string
describing the reason for a nonzero value; see @ref{Regexp Cleanup}.

@end deftypefun

Here are the possible nonzero values that @code{regcomp} can return:

@table @code
@comment regex.h
@comment POSIX.2
@item REG_BADBR
There was an invalid @samp{\@{@dots{}\@}} construct in the regular
expression.  A valid @samp{\@{@dots{}\@}} construct must contain either
a single number, or two numbers in increasing order separated by a
comma.

@comment regex.h
@comment POSIX.2
@item REG_BADPAT
There was a syntax error in the regular expression.

@comment regex.h
@comment POSIX.2
@item REG_BADRPT
A repetition operator such as @samp{?} or @samp{*} appeared in a bad
position (with no preceding subexpression to act on).

@comment regex.h
@comment POSIX.2
@item REG_ECOLLATE
The regular expression referred to an invalid collating element (one not
defined in the current locale for string collation).  @xref{Locale
Categories}.

@comment regex.h
@comment POSIX.2
@item REG_ECTYPE
The regular expression referred to an invalid character class name.

@comment regex.h
@comment POSIX.2
@item REG_EESCAPE
The regular expression ended with @samp{\}.

@comment regex.h
@comment POSIX.2
@item REG_ESUBREG
There was an invalid number in the @samp{\@var{digit}} construct.

@comment regex.h
@comment POSIX.2
@item REG_EBRACK
There were unbalanced square brackets in the regular expression.

@comment regex.h
@comment POSIX.2
@item REG_EPAREN
An extended regular expression had unbalanced parentheses,
or a basic regular expression had unbalanced @samp{\(} and @samp{\)}.

@comment regex.h
@comment POSIX.2
@item REG_EBRACE
The regular expression had unbalanced @samp{\@{} and @samp{\@}}.

@comment regex.h
@comment POSIX.2
@item REG_ERANGE
One of the endpoints in a range expression was invalid.

@comment regex.h
@comment POSIX.2
@item REG_ESPACE
@code{regcomp} ran out of memory.
@end table

@node Flags for POSIX Regexps
@subsection Flags for POSIX Regular Expressions

These are the bit flags that you can use in the @var{cflags} operand when
compiling a regular expression with @code{regcomp}.

@table @code
@comment regex.h
@comment POSIX.2
@item REG_EXTENDED
Treat the pattern as an extended regular expression, rather than as a
basic regular expression.

@comment regex.h
@comment POSIX.2
@item REG_ICASE
Ignore case when matching letters.

@comment regex.h
@comment POSIX.2
@item REG_NOSUB
Don't bother storing the contents of the @var{matches-ptr} array.

@comment regex.h
@comment POSIX.2
@item REG_NEWLINE
Treat a newline in @var{string} as dividing @var{string} into multiple
lines, so that @samp{$} can match before the newline and @samp{^} can
match after.  Also, don't permit @samp{.} to match a newline, and don't
permit @samp{[^@dots{}]} to match a newline.

Otherwise, newline acts like any other ordinary character.
@end table

@node Matching POSIX Regexps
@subsection Matching a Compiled POSIX Regular Expression

Once you have compiled a regular expression, as described in @ref{POSIX
Regexp Compilation}, you can match it against strings using
@code{regexec}.  A match anywhere inside the string counts as success,
unless the regular expression contains anchor characters (@samp{^} or
@samp{$}).

@comment regex.h
@comment POSIX.2
@deftypefun int regexec (regex_t *@var{compiled}, char *@var{string}, size_t @var{nmatch}, regmatch_t @var{matchptr} @t{[]}, int @var{eflags})
This function tries to match the compiled regular expression
@code{*@var{compiled}} against @var{string}.

@code{regexec} returns @code{0} if the regular expression matches;
otherwise, it returns a nonzero value.  See the table below for
what nonzero values mean.  You can use @code{regerror} to produce an
error message string describing the reason for a nonzero value;
see @ref{Regexp Cleanup}.

The argument @var{eflags} is a word of bit flags that enable various
options.

If you want to get information about what part of @var{string} actually
matched the regular expression or its subexpressions, use the arguments
@var{matchptr} and @var{nmatch}.  Otherwise, pass @code{0} for
@var{nmatch}, and @code{NULL} for @var{matchptr}.  @xref{Regexp
Subexpressions}.
@end deftypefun

You must match the regular expression with the same set of current
locales that were in effect when you compiled the regular expression.

The function @code{regexec} accepts the following flags in the
@var{eflags} argument:

@table @code
@comment regex.h
@comment POSIX.2
@item REG_NOTBOL
Do not regard the beginning of the specified string as the beginning of
a line; more generally, don't make any assumptions about what text might
precede it.

@comment regex.h
@comment POSIX.2
@item REG_NOTEOL
Do not regard the end of the specified string as the end of a line; more
generally, don't make any assumptions about what text might follow it.
@end table

Here are the possible nonzero values that @code{regexec} can return:

@table @code
@comment regex.h
@comment POSIX.2
@item REG_NOMATCH
The pattern didn't match the string.  This isn't really an error.

@comment regex.h
@comment POSIX.2
@item REG_ESPACE
@code{regexec} ran out of memory.
@end table

@node Regexp Subexpressions
@subsection Match Results with Subexpressions

When @code{regexec} matches parenthetical subexpressions of
@var{pattern}, it records which parts of @var{string} they match.  It
returns that information by storing the offsets into an array whose
elements are structures of type @code{regmatch_t}.  The first element of
the array (index @code{0}) records the part of the string that matched
the entire regular expression.  Each other element of the array records
the beginning and end of the part that matched a single parenthetical
subexpression.

@comment regex.h
@comment POSIX.2
@deftp {Data Type} regmatch_t
This is the data type of the @var{matcharray} array that you pass to
@code{regexec}.  It contains two structure fields, as follows:

@table @code
@item rm_so
The offset in @var{string} of the beginning of a substring.  Add this
value to @var{string} to get the address of that part.

@item rm_eo
The offset in @var{string} of the end of the substring.
@end table
@end deftp

@comment regex.h
@comment POSIX.2
@deftp {Data Type} regoff_t
@code{regoff_t} is an alias for another signed integer type.
The fields of @code{regmatch_t} have type @code{regoff_t}.
@end deftp

The @code{regmatch_t} elements correspond to subexpressions
positionally; the first element (index @code{1}) records where the first
subexpression matched, the second element records the second
subexpression, and so on.  The order of the subexpressions is the order
in which they begin.

When you call @code{regexec}, you specify how long the @var{matchptr}
array is, with the @var{nmatch} argument.  This tells @code{regexec} how
many elements to store.  If the actual regular expression has more than
@var{nmatch} subexpressions, then you won't get offset information about
the rest of them.  But this doesn't alter whether the pattern matches a
particular string or not.

If you don't want @code{regexec} to return any information about where
the subexpressions matched, you can either supply @code{0} for
@var{nmatch}, or use the flag @code{REG_NOSUB} when you compile the
pattern with @code{regcomp}.

@node Subexpression Complications
@subsection Complications in Subexpression Matching

Sometimes a subexpression matches a substring of no characters.  This
happens when @samp{f\(o*\)} matches the string @samp{fum}.  (It really
matches just the @samp{f}.)  In this case, both of the offsets identify
the point in the string where the null substring was found.  In this
example, the offsets are both @code{1}.

Sometimes the entire regular expression can match without using some of
its subexpressions at all---for example, when @samp{ba\(na\)*} matches the
string @samp{ba}, the parenthetical subexpression is not used.  When
this happens, @code{regexec} stores @code{-1} in both fields of the
element for that subexpression.

Sometimes matching the entire regular expression can match a particular
subexpression more than once---for example, when @samp{ba\(na\)*}
matches the string @samp{bananana}, the parenthetical subexpression
matches three times.  When this happens, @code{regexec} usually stores
the offsets of the last part of the string that matched the
subexpression.  In the case of @samp{bananana}, these offsets are
@code{6} and @code{8}.

But the last match is not always the one that is chosen.  It's more
accurate to say that the last @emph{opportunity} to match is the one
that takes precedence.  What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression.  For an example, consider @samp{\(ba\(na\)*s \)*} matching
the string @samp{bananas bas }.  The last time the inner expression
actually matches is near the end of the first word.  But it is
@emph{considered} again in the second word, and fails to match there.
@code{regexec} reports nonuse of the ``na'' subexpression.

Another place where this rule applies is when the regular expression
@smallexample
\(ba\(na\)*s \|nefer\(ti\)* \)*
@end smallexample
@noindent
matches @samp{bananas nefertiti}.  The ``na'' subexpression does match
in the first word, but it doesn't match in the second word because the
other alternative is used there.  Once again, the second repetition of
the outer subexpression overrides the first, and within that second
repetition, the ``na'' subexpression is not used.  So @code{regexec}
reports nonuse of the ``na'' subexpression.

@node Regexp Cleanup
@subsection POSIX Regexp Matching Cleanup

When you are finished using a compiled regular expression, you can
free the storage it uses by calling @code{regfree}.

@comment regex.h
@comment POSIX.2
@deftypefun void regfree (regex_t *@var{compiled})
Calling @code{regfree} frees all the storage that @code{*@var{compiled}}
points to.  This includes various internal fields of the @code{regex_t}
structure that aren't documented in this manual.

@code{regfree} does not free the object @code{*@var{compiled}} itself.
@end deftypefun

You should always free the space in a @code{regex_t} structure with
@code{regfree} before using the structure to compile another regular
expression.

When @code{regcomp} or @code{regexec} reports an error, you can use
the function @code{regerror} to turn it into an error message string.

@comment regex.h
@comment POSIX.2
@deftypefun size_t regerror (int @var{errcode}, regex_t *@var{compiled}, char *@var{buffer}, size_t @var{length})
This function produces an error message string for the error code
@var{errcode}, and stores the string in @var{length} bytes of memory
starting at @var{buffer}.  For the @var{compiled} argument, supply the
same compiled regular expression structure that @code{regcomp} or
@code{regexec} was working with when it got the error.  Alternatively,
you can supply @code{NULL} for @var{compiled}; you will still get a
meaningful error message, but it might not be as detailed.

If the error message can't fit in @var{length} bytes (including a
terminating null character), then @code{regerror} truncates it.
The string that @code{regerror} stores is always null-terminated
even if it has been truncated.

The return value of @code{regerror} is the minimum length needed to
store the entire error message.  If this is less than @var{length}, then
the error message was not truncated, and you can use it.  Otherwise, you
should call @code{regerror} again with a larger buffer.

Here is a function which uses @code{regerror}, but always dynamically
allocates a buffer for the error message:

@smallexample
char *get_regerror (int errcode, regex_t *compiled)
@{
  size_t length = regerror (errcode, compiled, NULL, 0);
  char *buffer = xmalloc (length);
  (void) regerror (errcode, compiled, buffer, length);
  return buffer;
@}
@end smallexample
@end deftypefun

@c !!!! this is not actually in the library....
@node Word Expansion
@section Shell-Style Word Expansion
@cindex word expansion
@cindex expansion of shell words

@dfn{Word expansion} means the process of splitting a string into
@dfn{words} and substituting for variables, commands, and wildcards
just as the shell does.

For example, when you write @samp{ls -l foo.c}, this string is split
into three separate words---@samp{ls}, @samp{-l} and @samp{foo.c}.
This is the most basic function of word expansion.

When you write @samp{ls *.c}, this can become many words, because
the word @samp{*.c} can be replaced with any number of file names.
This is called @dfn{wildcard expansion}, and it is also a part of
word expansion.

When you use @samp{echo $PATH} to print your path, you are taking
advantage of @dfn{variable substitution}, which is also part of word
expansion.

Ordinary programs can perform word expansion just like the shell by
calling the library function @code{wordexp}.

@menu
* Expansion Stages::	What word expansion does to a string.
* Calling Wordexp::	How to call @code{wordexp}.
* Flags for Wordexp::   Options you can enable in @code{wordexp}.
* Wordexp Example::	A sample program that does word expansion.
@end menu

@node Expansion Stages
@subsection The Stages of Word Expansion

When word expansion is applied to a sequence of words, it performs the
following transformations in the order shown here:

@enumerate
@item
@cindex tilde expansion
@dfn{Tilde expansion}: Replacement of @samp{~foo} with the name of
the home directory of @samp{foo}.

@item
Next, three different transformations are applied in the same step,
from left to right:

@itemize @bullet
@item
@cindex variable substitution
@cindex substitution of variables and commands
@dfn{Variable substitution}: Environment variables are substituted for
references such as @samp{$foo}.

@item
@cindex command substitution
@dfn{Command substitution}: Constructs such as @w{@samp{`cat foo`}} and
the equivalent @w{@samp{$(cat foo)}} are replaced with the output from
the inner command.

@item
@cindex arithmetic expansion
@dfn{Arithmetic expansion}: Constructs such as @samp{$(($x-1))} are
replaced with the result of the arithmetic computation.
@end itemize

@item
@cindex field splitting
@dfn{Field splitting}: subdivision of the text into @dfn{words}.

@item
@cindex wildcard expansion
@dfn{Wildcard expansion}: The replacement of a construct such as @samp{*.c}
with a list of @samp{.c} file names.  Wildcard expansion applies to an
entire word at a time, and replaces that word with 0 or more file names
that are themselves words.

@item
@cindex quote removal
@cindex removal of quotes
@dfn{Quote removal}: The deletion of string-quotes, now that they have
done their job by inhibiting the above transformations when appropriate.
@end enumerate

For the details of these transformations, and how to write the constructs
that use them, see @w{@cite{The BASH Manual}} (to appear).

@node Calling Wordexp
@subsection Calling @code{wordexp}

All the functions, constants and data types for word expansion are
declared in the header file @file{wordexp.h}.

Word expansion produces a vector of words (strings).  To return this
vector, @code{wordexp} uses a special data type, @code{wordexp_t}, which
is a structure.  You pass @code{wordexp} the address of the structure,
and it fills in the structure's fields to tell you about the results.

@comment wordexp.h
@comment POSIX.2
@deftp {Data Type} {wordexp_t}
This data type holds a pointer to a word vector.  More precisely, it
records both the address of the word vector and its size.

@table @code
@item we_wordc
The number of elements in the vector.

@item we_wordv
The address of the vector.  This field has type @w{@code{char **}}.

@item we_offs
The offset of the first real element of the vector, from its nominal
address in the @code{we_wordv} field.  Unlike the other fields, this
is always an input to @code{wordexp}, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of
the vector are left empty.  (The @code{wordexp} function fills them with
null pointers.)

The @code{we_offs} field is meaningful only if you use the
@code{WRDE_DOOFFS} flag.  Otherwise, the offset is always zero
regardless of what is in this field, and the first real element comes at
the beginning of the vector.
@end table
@end deftp

@comment wordexp.h
@comment POSIX.2
@deftypefun int wordexp (const char *@var{words}, wordexp_t *@var{word-vector-ptr}, int @var{flags})
Perform word expansion on the string @var{words}, putting the result in
a newly allocated vector, and store the size and address of this vector
into @code{*@var{word-vector-ptr}}.  The argument @var{flags} is a
combination of bit flags; see @ref{Flags for Wordexp}, for details of
the flags.

You shouldn't use any of the characters @samp{|&;<>} in the string
@var{words} unless they are quoted; likewise for newline.  If you use
these characters unquoted, you will get the @code{WRDE_BADCHAR} error
code.  Don't use parentheses or braces unless they are quoted or part of
a word expansion construct.  If you use quotation characters @samp{'"`},
they should come in pairs that balance.

The results of word expansion are a sequence of words.  The function
@code{wordexp} allocates a string for each resulting word, then
allocates a vector of type @code{char **} to store the addresses of
these strings.  The last element of the vector is a null pointer.
This vector is called the @dfn{word vector}.

To return this vector, @code{wordexp} stores both its address and its
length (number of elements, not counting the terminating null pointer)
into @code{*@var{word-vector-ptr}}.

If @code{wordexp} succeeds, it returns 0.  Otherwise, it returns one
of these error codes:

@table @code
@comment wordexp.h
@comment POSIX.2
@item WRDE_BADCHAR
The input string @var{words} contains an unquoted invalid character such
as @samp{|}.

@comment wordexp.h
@comment POSIX.2
@item WRDE_BADVAL
The input string refers to an undefined shell variable, and you used the flag
@code{WRDE_UNDEF} to forbid such references.

@comment wordexp.h
@comment POSIX.2
@item WRDE_CMDSUB
The input string uses command substitution, and you used the flag
@code{WRDE_NOCMD} to forbid command substitution.

@comment wordexp.h
@comment POSIX.2
@item WRDE_NOSPACE
It was impossible to allocate memory to hold the result.  In this case,
@code{wordexp} can store part of the results---as much as it could
allocate room for.

@comment wordexp.h
@comment POSIX.2
@item WRDE_SYNTAX
There was a syntax error in the input string.  For example, an unmatched
quoting character is a syntax error.
@end table
@end deftypefun

@comment wordexp.h
@comment POSIX.2
@deftypefun void wordfree (wordexp_t *@var{word-vector-ptr})
Free the storage used for the word-strings and vector that
@code{*@var{word-vector-ptr}} points to.  This does not free the
structure @code{*@var{word-vector-ptr}} itself---only the other
data it points to.
@end deftypefun

@node Flags for Wordexp
@subsection Flags for Word Expansion

This section describes the flags that you can specify in the
@var{flags} argument to @code{wordexp}.  Choose the flags you want,
and combine them with the C operator @code{|}.

@table @code
@comment wordexp.h
@comment POSIX.2
@item WRDE_APPEND
Append the words from this expansion to the vector of words produced by
previous calls to @code{wordexp}.  This way you can effectively expand
several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the
word vector structure between calls to @code{wordexp}.  And, if you set
@code{WRDE_DOOFFS} in the first call to @code{wordexp}, you must also
set it when you append to the results.

@comment wordexp.h
@comment POSIX.2
@item WRDE_DOOFFS
Leave blank slots at the beginning of the vector of words.
The @code{we_offs} field says how many slots to leave.
The blank slots contain null pointers.

@comment wordexp.h
@comment POSIX.2
@item WRDE_NOCMD
Don't do command substitution; if the input requests command substitution,
report an error.

@comment wordexp.h
@comment POSIX.2
@item WRDE_REUSE
Reuse a word vector made by a previous call to @code{wordexp}.
Instead of allocating a new vector of words, this call to @code{wordexp}
will use the vector that already exists (making it larger if necessary).

Note that the vector may move, so it is not safe to save an old pointer
and use it again after calling @code{wordexp}.  You must fetch
@code{we_pathv} anew after each call.

@comment wordexp.h
@comment POSIX.2
@item WRDE_SHOWERR
Do show any error messages printed by commands run by command substitution.
More precisely, allow these commands to inherit the standard error output
stream of the current process.  By default, @code{wordexp} gives these
commands a standard error stream that discards all output.

@comment wordexp.h
@comment POSIX.2
@item WRDE_UNDEF
If the input refers to a shell variable that is not defined, report an
error.
@end table

@node Wordexp Example
@subsection @code{wordexp} Example

Here is an example of using @code{wordexp} to expand several strings
and use the results to run a shell command.  It also shows the use of
@code{WRDE_APPEND} to concatenate the expansions and of @code{wordfree}
to free the space allocated by @code{wordexp}.

@smallexample
int
expand_and_execute (const char *program, const char *options)
@{
  wordexp_t result;
  pid_t pid
  int status, i;

  /* @r{Expand the string for the program to run.}  */
  switch (wordexp (program, &result, 0))
    @{
    case 0:			/* @r{Successful}.  */
      break;
    case WRDE_NOSPACE:
      /* @r{If the error was @code{WRDE_NOSPACE},}
         @r{then perhaps part of the result was allocated.}  */
      wordfree (&result);
    default:                    /* @r{Some other error.}  */
      return -1;
    @}

  /* @r{Expand the strings specified for the arguments.}  */
  for (i = 0; args[i]; i++)
    @{
      if (wordexp (options, &result, WRDE_APPEND))
        @{
          wordfree (&result);
          return -1;
        @}
    @}

  pid = fork ();
  if (pid == 0)
    @{
      /* @r{This is the child process.  Execute the command.} */
      execv (result.we_wordv[0], result.we_wordv);
      exit (EXIT_FAILURE);
    @}
  else if (pid < 0)
    /* @r{The fork failed.  Report failure.}  */
    status = -1;
  else
    /* @r{This is the parent process.  Wait for the child to complete.}  */
    if (waitpid (pid, &status, 0) != pid)
      status = -1;

  wordfree (&result);
  return status;
@}
@end smallexample


@c No sense finishing this for here.
@ignore
@node Tilde Expansion
@subsection Details of Tilde Expansion

It's a standard part of shell syntax that you can use @samp{~} at the
beginning of a file name to stand for your own home directory.  You
can use @samp{~@var{user}} to stand for @var{user}'s home directory.

@dfn{Tilde expansion} is the process of converting these abbreviations
to the directory names that they stand for.

Tilde expansion applies to the @samp{~} plus all following characters up
to whitespace or a slash.  It takes place only at the beginning of a
word, and only if none of the characters to be transformed is quoted in
any way.

Plain @samp{~} uses the value of the environment variable @code{HOME}
as the proper home directory name.  @samp{~} followed by a user name
uses @code{getpwname} to look up that user in the user database, and
uses whatever directory is recorded there.  Thus, @samp{~} followed
by your own name can give different results from plain @samp{~}, if
the value of @code{HOME} is not really your home directory.

@node Variable Substitution
@subsection Details of Variable Substitution

Part of ordinary shell syntax is the use of @samp{$@var{variable}} to
substitute the value of a shell variable into a command.  This is called
@dfn{variable substitution}, and it is one part of doing word expansion.

There are two basic ways you can write a variable reference for
substitution:

@table @code
@item $@{@var{variable}@}
If you write braces around the variable name, then it is completely
unambiguous where the variable name ends.  You can concatenate
additional letters onto the end of the variable value by writing them
immediately after the close brace.  For example, @samp{$@{foo@}s}
expands into @samp{tractors}.

@item $@var{variable}
If you do not put braces around the variable name, then the variable
name consists of all the alphanumeric characters and underscores that
follow the @samp{$}.  The next punctuation character ends the variable
name.  Thus, @samp{$foo-bar} refers to the variable @code{foo} and expands
into @samp{tractor-bar}.
@end table

When you use braces, you can also use various constructs to modify the
value that is substituted, or test it in various ways.

@table @code
@item $@{@var{variable}:-@var{default}@}
Substitute the value of @var{variable}, but if that is empty or
undefined, use @var{default} instead.

@item $@{@var{variable}:=@var{default}@}
Substitute the value of @var{variable}, but if that is empty or
undefined, use @var{default} instead and set the variable to
@var{default}.

@item $@{@var{variable}:?@var{message}@}
If @var{variable} is defined and not empty, substitute its value.

Otherwise, print @var{message} as an error message on the standard error
stream, and consider word expansion a failure.

@c ??? How does wordexp report such an error?

@item $@{@var{variable}:+@var{replacement}@}
Substitute @var{replacement}, but only if @var{variable} is defined and
nonempty.  Otherwise, substitute nothing for this construct.
@end table

@table @code
@item $@{#@var{variable}@}
Substitute a numeral which expresses in base ten the number of
characters in the value of @var{variable}.  @samp{$@{#foo@}} stands for
@samp{7}, because @samp{tractor} is seven characters.
@end table

These variants of variable substitution let you remove part of the
variable's value before substituting it.  The @var{prefix} and
@var{suffix} are not mere strings; they are wildcard patterns, just
like the patterns that you use to match multiple file names.  But
in this context, they match against parts of the variable value
rather than against file names.

@table @code
@item $@{@var{variable}%%@var{suffix}@}
Substitute the value of @var{variable}, but first discard from that
variable any portion at the end that matches the pattern @var{suffix}.

If there is more than one alternative for how to match against
@var{suffix}, this construct uses the longest possible match.

Thus, @samp{$@{foo%%r*@}} substitutes @samp{t}, because the largest
match for @samp{r*} at the end of @samp{tractor} is @samp{ractor}.

@item $@{@var{variable}%@var{suffix}@}
Substitute the value of @var{variable}, but first discard from that
variable any portion at the end that matches the pattern @var{suffix}.

If there is more than one alternative for how to match against
@var{suffix}, this construct uses the shortest possible alternative.

Thus, @samp{$@{foo%%r*@}} substitutes @samp{tracto}, because the shortest
match for @samp{r*} at the end of @samp{tractor} is just @samp{r}.

@item $@{@var{variable}##@var{prefix}@}
Substitute the value of @var{variable}, but first discard from that
variable any portion at the beginning that matches the pattern @var{prefix}.

If there is more than one alternative for how to match against
@var{prefix}, this construct uses the longest possible match.

Thus, @samp{$@{foo%%r*@}} substitutes @samp{t}, because the largest
match for @samp{r*} at the end of @samp{tractor} is @samp{ractor}.

@item $@{@var{variable}#@var{prefix}@}
Substitute the value of @var{variable}, but first discard from that
variable any portion at the beginning that matches the pattern @var{prefix}.

If there is more than one alternative for how to match against
@var{prefix}, this construct uses the shortest possible alternative.

Thus, @samp{$@{foo%%r*@}} substitutes @samp{tracto}, because the shortest
match for @samp{r*} at the end of @samp{tractor} is just @samp{r}.

@end ignore