GAWK: Effective AWK Programming

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210 GAWK: Effective AWK Programminguser database, and the I/O to scan it, are only incurred if the user’s main program actuallycalls one of these functions. If this library file is loaded along with a user’s program, but noneof the routines are ever called, then there is no extra runtime overhead. (The alternativeis move the body of _pw_init into a BEGIN rule, which always runs pwcat. This simplifiesthe code but runs an extra process that may never be needed.)In turn, calling _pw_init is not too expensive, because the _pw_inited variable keepsthe program from reading the data more than once. If you are worried about squeezingevery last cycle out of your awk program, the check of _pw_inited could be moved out of_pw_init and duplicated in all the other functions. In practice, this is not necessary, sincemost awk programs are I/O-bound, and it clutters up the code.The id program in Section 13.2.3 [Printing out User Information], page 224, uses thesefunctions.12.6 Reading the Group DatabaseMuch of the discussion presented in Section 12.5 [Reading the User Database], page 206,applies to the group database as well. Although there has traditionally been a well-knownfile (‘/etc/group’) in a well-known format, the POSIX standard only provides a set of Clibrary routines ( and getgrent) for accessing the information. Even though thisfile may exist, it likely does not have complete information. Therefore, as with the userdatabase, it is necessary to have a small C program that generates the group database asits output.grcat, a C program that “cats” the group database, is as follows:/** grcat.c** Generate a printable version of the group database*/#include #include intmain(argc, argv)int argc;char **argv;{struct group *g;int i;while ((g = getgrent()) != NULL) {printf("%s:%s:%ld:", g->gr_name, g->gr_passwd,(long) g->gr_gid);for (i = 0; g->gr_mem[i] != NULL; i++) {printf("%s", g->gr_mem[i]);if (g->gr_mem[i+1] != NULL)putchar(’,’);}
Chapter 12: A Library of awk Functions 211putchar(’\n’);}endgrent();return 0;}Each line in the group database represents one group. The fields are separated withcolons and represent the following information:Group name The group’s name.Group passwordGroup-IDThe group’s encrypted password. In practice, this fieldis never used; it is usually empty or set to ‘*’.The group’s numeric group ID number; this numbershould be unique within the file.Group member list A comma-separated list of user names. These usersare members of the group. Modern Unix systems allowusers to be members of several groups simultaneously.If your system does, then there are elements"group1" through "groupN" in PROCINFO for thosegroup ID numbers. (Note that PROCINFO is a gawk extension;see Section 6.5 [Built-in Variables], page 110.)Here is what running grcat might produce:$ grcat⊣ wheel:*:0:arnold⊣ nogroup:*:65534:⊣ daemon:*:1:⊣ kmem:*:2:⊣ staff:*:10:arnold,miriam,andy⊣ other:*:20:...Here are the functions for obtaining information from the group database. There areseveral, modeled after the C library functions of the same names:# group.awk --- functions for dealing with the group fileBEGIN \{# Change to suit your system_gr_awklib = "/usr/local/libexec/awk/"}function _gr_init({if (_gr_inited)returnoldfs, oldrs, olddol0, grcat,using_fw, n, a, i)
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