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(argc, argv)
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.
|
| Group-ID | The user's numeric group ID number.
|
| 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)
{
if (_pw_inited)
return
oldfs = FS
oldrs = RS
olddol0 = $0
using_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 = 1
FS = oldfs
if (using_fw)
FIELDWIDTHS = FIELDWIDTHS
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; _pw_init
needs only to be called 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 on some other
awk 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 cleans up by closing the pipeline,
setting _pw_inited to one, and restoring FS (and FIELDWIDTHS
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
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 number is 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 getpwent start over again:
function endpwent()
{
_pw_count = 0
}
A conscious design decision in this suite was made 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 it
clutters up the code.
The id program in Id Program, uses these functions.