Unix Environment Variables

Let’s talk about some popular unix environment variables:

• $USER - The current user
• $PAGER - the program that accepts page by page input. less and more are good examples.
• $VISUAL - A full screen editor (like vi, emacs, and nano).
• $EDITOR - A line by line editor (ed or ex work).
• $PWD - the current working directory
• $HOME - the home directory
• $LANG - the language you use, with an optional encoding.
• $MANPATH - the list of directories to search for manual pages.
• $MAIL - where mail goes
• $SHELL - path to shell binary you use (e.g. /bin/bash, /bin/ksh, /bin/sh, /bin/zsh)

The most important one is probably $PATH, which is where the OS looks for binaries. It goes from the beginning to the end, executing the first binary it finds.

Let’s say my $PATH is like this:

/usr/local/bin:/usr/bin

Which instructs the OS to look into /usr/local/bin to find a valid binary. Then /usr/bin.

The /bin directory contains binaries for sysadmins and users, but are required when there’s no filesystem in use.

The /usr/bin and was meant to contain executable programs that were part of the OS

and /usr/local/bin is for software that the user installs.

There directories where superuser binaries should be located which follow the same scheme:

• /sbin
• /usr/sbin
• /usr/local/sbin

As well, /usr/share/bin is for binaries used for web servers and clients.

If you find that a command doesn’t work, double-check to make sure that your $PATH is set up properly to find the correct binary.