GAWK: Effective AWK Programming

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224 GAWK: Effective AWK Programming}e = "Usage: egrep [-csvil] [-e pat] [files ...]"e = e "\n\tegrep [-csvil] pat [files ...]"print e > "/dev/stderr"exit 1The variable e is used so that the function fits nicely on the printed page.Just a note on programming style: you may have noticed that the END rule uses backslashcontinuation, with the open brace on a line by itself. This is so that it more closely resemblesthe way functions are written. Many of the examples in this chapter use this style. Youcan decide for yourself if you like writing your BEGIN and END rules this way or not.13.2.3 Printing out User InformationThe id utility lists a user’s real and effective user ID numbers, real and effective group IDnumbers, and the user’s group set, if any. id only prints the effective user ID and group IDif they are different from the real ones. If possible, id also supplies the corresponding userand group names. The output might look like this:$ id⊣ uid=2076(arnold) gid=10(staff) groups=10(staff),4(tty)This information is part of what is provided by gawk’s PROCINFO array (see Section 6.5[Built-in Variables], page 110). However, the id utility provides a more palatable outputthan just individual numbers.Here is a simple version of id written in awk. It uses the user database library functions(see Section 12.5 [Reading the User Database], page 206) and the group database libraryfunctions (see Section 12.6 [Reading the Group Database], page 210):The program is fairly straightforward. All the work is done in the BEGIN rule. The userand group ID numbers are obtained from PROCINFO. The code is repetitive. The entry inthe user database for the real user ID number is split into parts at the ‘:’. The name is thefirst field. Similar code is used for the effective user ID number and the group numbers:# id.awk --- implement id in awk## Requires user and group library functions# output is:# uid=12(foo) euid=34(bar) gid=3(baz) \# egid=5(blat) groups=9(nine),2(two),1(one)BEGIN \{uid = PROCINFO["uid"]euid = PROCINFO["euid"]gid = PROCINFO["gid"]egid = PROCINFO["egid"]printf("uid=%d", uid)pw = getpwuid(uid)if (pw != "") {
Chapter 13: Practical awk Programs 225}split(pw, a, ":")printf("(%s)", a[1])if (euid != uid) {printf(" euid=%d", euid)pw = getpwuid(euid)if (pw != "") {split(pw, a, ":")printf("(%s)", a[1])}}printf(" gid=%d", gid)pw = getgrgid(gid)if (pw != "") {split(pw, a, ":")printf("(%s)", a[1])}if (egid != gid) {printf(" egid=%d", egid)pw = getgrgid(egid)if (pw != "") {split(pw, a, ":")printf("(%s)", a[1])}}for (i = 1; ("group" i) in PROCINFO; i++) {if (i == 1)printf(" groups=")group = PROCINFO["group" i]printf("%d", group)pw = getgrgid(group)if (pw != "") {split(pw, a, ":")printf("(%s)", a[1])}if (("group" (i+1)) in PROCINFO)printf(",")}}print ""
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