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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
><TITLE
>Recursion: a script calling itself</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+
"><LINK
REL="HOME"
TITLE="Advanced Bash-Scripting Guide"
HREF="index.html"><LINK
REL="UP"
TITLE="Miscellany"
HREF="miscellany.html"><LINK
REL="PREVIOUS"
TITLE="Tests and Comparisons: Alternatives"
HREF="testsandcomparisons.html"><LINK
REL="NEXT"
TITLE="Colorizing Scripts"
HREF="colorizing.html"><META
HTTP-EQUIV="Content-Style-Type"
CONTENT="text/css"><LINK
REL="stylesheet"
HREF="common/kde-common.css"
TYPE="text/css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=iso-8859-1"><META
HTTP-EQUIV="Content-Language"
CONTENT="en"><LINK
REL="stylesheet"
HREF="common/kde-localised.css"
TYPE="text/css"
TITLE="KDE-English"><LINK
REL="stylesheet"
HREF="common/kde-default.css"
TYPE="text/css"
TITLE="KDE-Default"></HEAD
><BODY
CLASS="SECT1"
BGCOLOR="#FFFFFF"
TEXT="#000000"
LINK="#AA0000"
VLINK="#AA0055"
ALINK="#AA0000"
STYLE="font-family: sans-serif;"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="3"
ALIGN="center"
>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="testsandcomparisons.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="80%"
ALIGN="center"
VALIGN="bottom"
>Chapter 36. Miscellany</TD
><TD
WIDTH="10%"
ALIGN="right"
VALIGN="bottom"
><A
HREF="colorizing.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RECURSIONSCT"
></A
>36.4. Recursion: a script calling itself</H1
><P
><A
NAME="SCRIPTRECURSION"
></A
></P
><P
>Can a script <A
HREF="localvar.html#RECURSIONREF"
>recursively</A
>
call itself? Indeed.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RECURSE"
></A
><P
><B
>Example 36-10. A (useless) script that recursively calls itself</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> 1 #!/bin/bash
2 # recurse.sh
3
4 # Can a script recursively call itself?
5 # Yes, but is this of any practical use?
6 # (See the following.)
7
8 RANGE=10
9 MAXVAL=9
10
11 i=$RANDOM
12 let "i %= $RANGE" # Generate a random number between 0 and $RANGE - 1.
13
14 if [ "$i" -lt "$MAXVAL" ]
15 then
16 echo "i = $i"
17 ./$0 # Script recursively spawns a new instance of itself.
18 fi # Each child script does the same, until
19 #+ a generated $i equals $MAXVAL.
20
21 # Using a "while" loop instead of an "if/then" test causes problems.
22 # Explain why.
23
24 exit 0
25
26 # Note:
27 # ----
28 # This script must have execute permission for it to work properly.
29 # This is the case even if it is invoked by an "sh" command.
30 # Explain why.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="PBOOK"
></A
><P
><B
>Example 36-11. A (useful) script that recursively calls itself</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> 1 #!/bin/bash
2 # pb.sh: phone book
3
4 # Written by Rick Boivie, and used with permission.
5 # Modifications by ABS Guide author.
6
7 MINARGS=1 # Script needs at least one argument.
8 DATAFILE=./phonebook
9 # A data file in current working directory
10 #+ named "phonebook" must exist.
11 PROGNAME=$0
12 E_NOARGS=70 # No arguments error.
13
14 if [ $# -lt $MINARGS ]; then
15 echo "Usage: "$PROGNAME" data-to-look-up"
16 exit $E_NOARGS
17 fi
18
19
20 if [ $# -eq $MINARGS ]; then
21 grep $1 "$DATAFILE"
22 # 'grep' prints an error message if $DATAFILE not present.
23 else
24 ( shift; "$PROGNAME" $* ) | grep $1
25 # Script recursively calls itself.
26 fi
27
28 exit 0 # Script exits here.
29 # Therefore, it's o.k. to put
30 #+ non-hashmarked comments and data after this point.
31
32 # ------------------------------------------------------------------------
33 Sample "phonebook" datafile:
34
35 John Doe 1555 Main St., Baltimore, MD 21228 (410) 222-3333
36 Mary Moe 9899 Jones Blvd., Warren, NH 03787 (603) 898-3232
37 Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
38 Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
39 Zoe Zenobia 4481 N. Baker St., San Francisco, SF 94338 (415) 501-1631
40 # ------------------------------------------------------------------------
41
42 $bash pb.sh Roe
43 Richard Roe 856 E. 7th St., New York, NY 10009 (212) 333-4567
44 Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
45
46 $bash pb.sh Roe Sam
47 Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
48
49 # When more than one argument is passed to this script,
50 #+ it prints *only* the line(s) containing all the arguments.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="USRMNT"
></A
><P
><B
>Example 36-12. Another (useful) script that recursively calls itself</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> 1 #!/bin/bash
2 # usrmnt.sh, written by Anthony Richardson
3 # Used in ABS Guide with permission.
4
5 # usage: usrmnt.sh
6 # description: mount device, invoking user must be listed in the
7 # MNTUSERS group in the /etc/sudoers file.
8
9 # ----------------------------------------------------------
10 # This is a usermount script that reruns itself using sudo.
11 # A user with the proper permissions only has to type
12
13 # usermount /dev/fd0 /mnt/floppy
14
15 # instead of
16
17 # sudo usermount /dev/fd0 /mnt/floppy
18
19 # I use this same technique for all of my
20 #+ sudo scripts, because I find it convenient.
21 # ----------------------------------------------------------
22
23 # If SUDO_COMMAND variable is not set we are not being run through
24 #+ sudo, so rerun ourselves. Pass the user's real and group id . . .
25
26 if [ -z "$SUDO_COMMAND" ]
27 then
28 mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*
29 exit 0
30 fi
31
32 # We will only get here if we are being run by sudo.
33 /bin/mount $* -o uid=$mntusr,gid=$grpusr
34
35 exit 0
36
37 # Additional notes (from the author of this script):
38 # -------------------------------------------------
39
40 # 1) Linux allows the "users" option in the /etc/fstab
41 # file so that any user can mount removable media.
42 # But, on a server, I like to allow only a few
43 # individuals access to removable media.
44 # I find using sudo gives me more control.
45
46 # 2) I also find sudo to be more convenient than
47 # accomplishing this task through groups.
48
49 # 3) This method gives anyone with proper permissions
50 # root access to the mount command, so be careful
51 # about who you allow access.
52 # You can get finer control over which access can be mounted
53 # by using this same technique in separate mntfloppy, mntcdrom,
54 # and mntsamba scripts.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Too many levels of recursion can exhaust the
script's stack space, causing a segfault.</P
></TD
></TR
></TABLE
></DIV
></DIV
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="testsandcomparisons.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="colorizing.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Tests and Comparisons: Alternatives</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="miscellany.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><SPAN
CLASS="QUOTE"
>"Colorizing"</SPAN
> Scripts</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>
|
