Shell Environment – What Sets the $SHELL Environment Variable?

An environment is not as magical as it might seem. The shell stores it in memory and passes to the execve() system call. The child process inherits it as an array pointer called environ. From the execve manpage:

SYNOPSIS

   #include <unistd.h>

   int execve(const char *filename, char *const argv[],
              char *const envp[]);

argv is an array of argument strings passed to the new program.
By convention, the first of these strings should contain the filename associated with the file being executed. envp is an array of strings, conventionally of the form key=value, which are passed as environment to the new program.

The environ(7) manpage also offers some insight:

SYNOPSIS

   extern char **environ;

DESCRIPTION

The variable environ points to an array of pointers to strings called the "environment". The last pointer in this array has the value NULL. (This variable must be declared in the user program, but is declared in the header file <unistd.h> in case the header files came from libc4 or libc5, and in case they came from glibc and _GNU_SOURCE was defined.) This array of strings is made available to the process by the exec(3) call that started the process.

Both of these GNU manpages match the POSIX specification

If all the shells you're interested in are Bourne-compatible, you can use /etc/profile for this purpose.

The header of /etc/profile:

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...)

To ensure you've got csh and tcsh covered, you can also add your variables to /etc/csh.login.

The header of /etc/csh.login:

# /etc/csh.login: system-wide .login file for csh(1) and tcsh(1)

For zsh, you want /etc/zshenv.

For ease of maintenance

I would write all the variables you want in a single file and write a simple Perl (or other) script that would read these variables and update the relevant files for all the shells.

Something like the following in Perl should work:

 #!/usr/bin/perl

use strict;
use warnings;

my $bourne_file = '/etc/profile';
my $csh_file = '/etc/csh.login';
my $zsh_file = '/etc/zshenv';
open my $BOURNE,'>>', $bourne_file;
open my $CSH,'>>', $csh_file;
open my $ZSH,'>>', $zsh_file;
while(<DATA>){
    chomp;
    my $delimiter = ','; #Change , to whatever delimiter you use in the file

    my ($var, @value) = split /$delimiter/;        
    my $value = join $delimiter,@value;
    print $BOURNE qq{export $var="$value"\n};
    print $CSH    qq{setenv $var "$value"\n};
    print $ZSH    qq{export $var="$value"\n};
}
close $BOURNE;
close $CSH;
close $ZSH;
__DATA__
var1,val1
var2,val2

You can use a delimiter other than , to delimit variables and values as long as this delimiter isn't allowed in variable names.

This can be further tidied up by inserting unique delimiters around the portion you want your script to write in each file so that each time you use the script to update the files, it doesn't duplicate previous entries but substitutes them in situ.