GAWK: Effective AWK Programming

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(’,’);}