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10.5 Reading the User Database

The PROCINFO array (see Built-in Variables) provides access to the current user's real and effective user and group ID numbers, and if available, the user's supplementary group set. However, because these are numbers, they do not provide very useful information to the average user. There needs to be some way to find the user information associated with the user and group ID numbers. This section presents a suite of functions for retrieving information from the user database. See Group Functions, for a similar suite that retrieves information from the group database.

The POSIX standard does not define the file where user information is kept. Instead, it provides the <pwd.h> header file and several C language subroutines for obtaining user information. The primary function is getpwent(), for “get password entry.” The “password” comes from the original user database file, /etc/passwd, which stores user information, along with the encrypted passwords (hence the name).

While an awk program could simply read /etc/passwd directly, this file may not contain complete information about the system's set of users.1 To be sure you are able to produce a readable and complete version of the user database, it is necessary to write a small C program that calls getpwent(). getpwent() is defined as returning a pointer to a struct passwd. Each time it is called, it returns the next entry in the database. When there are no more entries, it returns NULL, the null pointer. When this happens, the C program should call endpwent() to close the database. Following is pwcat, a C program that “cats” the password database:

     
     /*
      * pwcat.c
      *
      * Generate a printable version of the password database
      */
     
     
     #include <stdio.h>
     #include <pwd.h>
     
     
     
     int
     main(int argc, char **argv)
     {
         struct passwd *p;
     
         while ((p = getpwent()) != NULL)
     
     
             printf("%s:%s:%ld:%ld:%s:%s:%s\n",
                 p->pw_name, p->pw_passwd, (long) p->pw_uid,
                 (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
     
     
     
         endpwent();
         return 0;
     }
     

If you don't understand C, don't worry about it. The output from pwcat is the user database, in the traditional /etc/passwd format of colon-separated fields. The fields are:

Login name
The user's login name.
Encrypted password
The user's encrypted password. This may not be available on some systems.
User-ID
The user's numeric user ID number. (On some systems it's a C long, and not an int. Thus we cast it to long for all cases.)
Group-ID
The user's numeric group ID number. (Similar comments about long vs. int apply here.)
Full name
The user's full name, and perhaps other information associated with the user.
Home directory
The user's login (or “home”) directory (familiar to shell programmers as $HOME).
Login shell
The program that is run when the user logs in. This is usually a shell, such as Bash.

A few lines representative of pwcat's output are as follows:

     $ pwcat
     -| root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh
     -| nobody:*:65534:65534::/:
     -| daemon:*:1:1::/:
     -| sys:*:2:2::/:/bin/csh
     -| bin:*:3:3::/bin:
     -| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
     -| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
     -| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
     ...

With 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 information
     
     
     
     BEGIN {
         # tailor this to suit your system
         _pw_awklib = "/usr/local/libexec/awk/"
     }
     
     function _pw_init(    oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)
     {
         if (_pw_inited)
             return
     
         oldfs = FS
         oldrs = RS
         olddol0 = $0
         using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
         using_fpat = (PROCINFO["FS"] == "FPAT")
         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 = 1
         FS = oldfs
         if (using_fw)
             FIELDWIDTHS = FIELDWIDTHS
         else if (using_fpat)
             FPAT = FPAT
         RS = oldrs
         $0 = olddol0
     }
     

The 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 on your system.

The function _pw_init() keeps three copies of the user information in three associative arrays. 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, since _pw_init() needs to be called only once.

Because this function uses getline to read information from pwcat, it first saves the values of FS, RS, and $0. It notes in the variable using_fw whether field splitting with FIELDWIDTHS is in effect or not. Doing so is necessary, since these functions could be called from anywhere within a user's program, and the user may have his or her own way of splitting records and fields.

The using_fw variable checks PROCINFO["FS"], which is "FIELDWIDTHS" if field splitting is being done with FIELDWIDTHS. This makes it possible to restore the correct field-splitting mechanism later. The test can only be true for gawk. It is false if using FS or FPAT, or on some other awk implementation.

The code that checks for using FPAT, using using_fpat and PROCINFO["FS"] is similar.

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() cleans up by closing the pipeline, setting _pw_inited to one, and restoring FS (and FIELDWIDTHS or FPAT if 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 the database, it returns the appropriate line. Otherwise, it relies on the array reference to a nonexistent element to create the element with the null string as its value:

     
     function getpwnam(name)
     {
         _pw_init()
         return _pw_byname[name]
     }
     

Similarly, the getpwuid function takes a user ID number argument. If that user number is in the database, it returns the appropriate line. Otherwise, it returns the null string:

     
     function getpwuid(uid)
     {
         _pw_init()
         return _pw_byuid[uid]
     }
     

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 getpwent() start over again:

     
     function endpwent()
     {
         _pw_count = 0
     }
     

A conscious design decision in this suite is that each subroutine calls _pw_init() to initialize the database arrays. The overhead of running a separate process to generate the user database, and the I/O to scan it, are only incurred if the user's main program actually calls one of these functions. If this library file is loaded along with a user's program, but none of the routines are ever called, then there is no extra runtime overhead. (The alternative is move the body of _pw_init() into a BEGIN rule, which always runs pwcat. This simplifies the 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 keeps the program from reading the data more than once. If you are worried about squeezing every 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, since most awk programs are I/O-bound, and such a change would clutter up the code.

The id program in Id Program, uses these functions.


Footnotes

[1] It is often the case that password information is stored in a network database.