GAWK: Effective AWK Programming

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208 GAWK: Effective AWK ProgrammingWith that introduction, following is a group of functions for getting user information.There are several functions here, corresponding to the C functions of the same names:# passwd.awk --- access password file informationBEGIN {# tailor this to suit your system_pw_awklib = "/usr/local/libexec/awk/"}function _pw_init({if (_pw_inited)returnoldfs, oldrs, olddol0, pwcat, using_fw)}oldfs = FSoldrs = RSolddol0 = $0using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")FS = ":"RS = "\n"pwcat = _pw_awklib "pwcat"while ((pwcat | getline) > 0) {_pw_byname[$1] = $0_pw_byuid[$3] = $0_pw_bycount[++_pw_total] = $0}close(pwcat)_pw_count = 0_pw_inited = 1FS = oldfsif (using_fw)FIELDWIDTHS = FIELDWIDTHSRS = oldrs$0 = olddol0The BEGIN rule sets a private variable to the directory where pwcat is stored.Because it is used to help out an awk library routine, we have chosen to put it in‘/usr/local/libexec/awk’; however, you might want it to be in a different directory onyour system.The function _pw_init keeps three copies of the user information in three associativearrays. The arrays are indexed by username (_pw_byname), by user ID number (_pw_byuid),and by order of occurrence (_pw_bycount). The variable _pw_inited is used for efficiency;_pw_init needs only to be called once.Because this function uses getline to read information from pwcat, it first saves thevalues of FS, RS, and $0. It notes in the variable using_fw whether field splitting withFIELDWIDTHS is in effect or not. Doing so is necessary, since these functions could be called
Chapter 12: A Library of awk Functions 209from anywhere within a user’s program, and the user may have his or her own way ofsplitting records and fields.The using_fw variable checks PROCINFO["FS"], which is "FIELDWIDTHS" if field splittingis being done with FIELDWIDTHS. This makes it possible to restore the correct field-splittingmechanism later. The test can only be true for gawk. It is false if using FS or on some otherawk implementation.The main part of the function uses a loop to read database lines, split the line into fields,and then store the line into each array as necessary. When the loop is done, _pw_init cleansup by closing the pipeline, setting _pw_inited to one, and restoring FS (and FIELDWIDTHSif necessary), RS, and $0. The use of _pw_count is explained shortly.The getpwnam function takes a username as a string argument. If that user is in thedatabase, it returns the appropriate line. Otherwise, it returns the null string:function getpwnam(name){_pw_init()if (name in _pw_byname)return _pw_byname[name]return ""}Similarly, the getpwuid function takes a user ID number argument. If that user numberis in the database, it returns the appropriate line. Otherwise, it returns the null string:function getpwuid(uid){_pw_init()if (uid in _pw_byuid)return _pw_byuid[uid]return ""}The getpwent function simply steps through the database, one entry at a time. It uses_pw_count to track its current position in the _pw_bycount array:function getpwent(){_pw_init()if (_pw_count < _pw_total)return _pw_bycount[++_pw_count]return ""}The endpwent function resets _pw_count to zero, so that subsequent calls to getpwentstart over again:function endpwent(){_pw_count = 0}A conscious design decision in this suite was made that each subroutine calls _pw_initto initialize the database arrays. The overhead of running a separate process to generate the
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GAWK: Effective AWK Programming

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