Intro To 'printenv' Command In Linux

2024-02-28 - By Robert Elder

I use the 'printenv' command to print environment variables:

printenv

LESSCLOSE=/usr/bin/lesspipe %s %s
XDG_SESSION_CLASS=user
TERM=xterm-256color
LESSOPEN=| /usr/bin/lesspipe %s
USER=robert
DISPLAY=:0
SHLVL=1
QT_IM_MODULE=ibus
XDG_RUNTIME_DIR=/run/user/1000
...

Print Select Environment Variables

By default, 'printenv' prints all environment variables, but you can also specify a select subset:

printenv DISPLAY TERM

:0
xterm-256color

Origin Of 'printenv' Command

The 'printenv' command first appeared in 1979 as a contribution by Bill Joy, to the Berkeley Software Distribution:

/*
 * Copyright (c) 1980 Regents of the University of California.
 * All rights reserved.  The Berkeley software License Agreement
 * specifies the terms and conditions for redistribution.
 */

#ifndef lint
char copyright[] =
"@(#) Copyright (c) 1980 Regents of the University of California.\n\
 All rights reserved.\n";
#endif not lint

#ifndef lint
static char sccsid[] = "@(#)printenv.c	5.1 (Berkeley) 5/31/85";
#endif not lint

/*
 * printenv
 *
 * Bill Joy, UCB
 * February, 1979
 */

extern	char **environ;

main(argc, argv)
	int argc;
	char *argv[];
{
	register char **ep;
	int found = 0;

	argc--, argv++;
	if (environ)
		for (ep = environ; *ep; ep++)
			if (argc == 0 || prefix(argv[0], *ep)) {
				register char *cp = *ep;

				found++;
				if (argc) {
					while (*cp && *cp != '=')
						cp++;
					if (*cp == '=')
						cp++;
				}
				printf("%s\n", cp);
			}
	exit (!found);
}

prefix(cp, dp)
	char *cp, *dp;
{

	while (*cp && *dp && *cp == *dp)
		cp++, dp++;
	if (*cp == 0)
		return (*dp == '=');
	return (0);
}

The source code above was found at printenv.c.

The 'printenv' Command Versus 'env'

The purpose of 'printenv' is exclusively to print environment variables, which can also be done by the 'env' command as well:

env

LESSCLOSE=/usr/bin/lesspipe %s %s
XDG_SESSION_CLASS=user
TERM=xterm-256color
LESSOPEN=| /usr/bin/lesspipe %s
USER=robert
DISPLAY=:0
SHLVL=1
QT_IM_MODULE=ibus
XDG_RUNTIME_DIR=/run/user/1000
...

However, the 'env' command also includes a number of other features that the 'printenv' command doesn't, making 'printenv' effectively redundant.

The reason that the 'printenv' command exists can likely be attributed to the fact that the 'env' command was created independently around the same time, and appeared as early as 1980 in Unix release version 3.

In subsequent years, the GNU software collection implemented support for both the 'printenv' command and the 'env' command:

#  https://github.com/coreutils/coreutils.git
git log --raw ccbd1 src/env.c src/printenv.c

commit ccbd1d7dc5189f4637468a8136f672e60ee0e531
Author: Jim Meyering <jim@meyering.net>
Date:   Sun Nov 1 05:44:29 1992 +0000

    Initial revision

:000000 100644 000000000 b322dee60 A    src/env.c
:000000 100644 000000000 ac4511cc5 A    src/printenv.c

These eventually became part of Shell Utils, and finally Core Utils.

And that's why the 'printenv' command is my favourite Linux command.

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