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Linux Server Hacks by Rob Flickenger

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Hack #10. At Home in Your Shell Environment

Make bash more comfortable through environment variables

Consulting a manpage for bash can be a daunting read, especially if you're not precisely sure what you're looking for. But when you have the time to devote to it, the manpage for bash is well worth the read. This is a shell just oozing with all sorts of arcane (but wonderfully useful) features, most of which are simply disabled by default.

Let's start by looking at some useful environment variables, and some useful values to which to set them:

export PS1=`echo -ne "\033[0;34m\u@\h:\033[0;36m\w\033[0;34m\$\033[0;37m "`

As you probably know, the PS1 variable sets the default system prompt, and automatically interprets escape sequences such as \u (for username) and \w (for the current working directory.) As you may not know, it is possible to encode ANSI escape sequences in your shell prompt, to give your prompt a colorized appearance. We wrap the whole string in backticks (` ) in order to get echo to generate the magic ASCII escape character. This is executed once, and the result is stored in PS1. Let's look at that line again, with boldface around everything that isn't an ANSI code:

export PS1=`echo -ne "\033[0;34m\u@\h:\033[0;36m\w\033[0;34m\$\033[0;37m "`

You should recognize the familiar \u@\h:\w\$prompt that we've all grown to know and love. By changing the numbers just after each semicolon, you can set the colors of each part of the prompt to your heart's content.

Along the same lines, here's a handy command that is run just before bash gives you a prompt:

export PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME}: ${PWD}\007"'

(We don't need backticks for this one, as bash is expecting it to contain an actual command, not a string.) This time, the escape sequence is the magic string that manipulates the titlebar on most terminal windows (such as xterm, rxvt, eterm, gnometerm, etc.). Anything after the semicolon and before the \007 gets printed to your titlebar every time you get a new prompt. In this case, we're displaying your username, the host you're logged into, and the current working directory. This is quite handy for being able to tell at a glance (or even while within vim) to which machine you're logged in, and to what directory you're about to save your file. See [Hack #59] if you'd like to update your titlebar in real time instead of at every new bash prompt.

Have you ever accidentally hit ^D too many times in a row, only to find yourself logged out? You can tell bash to ignore as many consecutive ^D hits as you like:

export IGNOREEOF=2

This makes bash follow the Snark rule ("What I tell you three times is true") and only log you out if you hit ^D three times in a row. If that's too few for you, feel free to set it to 101 and bash will obligingly keep count for you.

Having a directory just off of your home that lies in your path can be extremely useful (for keeping scripts, symlinks, and other random pieces of code.) A traditional place to keep this directory is in bin underneath your home directory. If you use the ~ expansion facility in bash, like this:

export PATH=$PATH:~/bin

then the path will always be set properly, even if your home directory ever gets moved (or if you decide you want to use this same line on multiple machines with potentially different home directories — as in movein.sh). See [Hack #72].

Did you know that just as commands are searched for in the PATH variable (and manpages are searched for in the MANPATH variable), directories are likewise searched for in the CDPATH variable every time you issue a cd? By default, it is only set to ".", but can be set to anything you like:

export CDPATH=.:~

This will make cd search not only the current directory, but also your home directory for the directory you try to change to. For example:

rob@caligula:~$ ls
bin/ devel/ incoming/ mail/ test/ stuff.txt
rob@caligula:~$ cd /usr/local/bin
rob@caligula:/usr/local/bin$ cd mail
bash: cd: mail: No such file or directory
rob@caligula:/usr/local/bin$ export CDPATH=.:~
rob@caligula:/usr/local/bin$ cd mail
/home/rob/mail
rob@caligula:~/mail$

You can put as many paths as you like to search for in CDPATH, separating each with a : (just as with the PATH and MANPATH variables.)

We all know about the up arrow and the history command. But what happens if you accidentally type something sensitive on the command line? Suppose you slip while typing and accidentally type a password where you meant to type a command. This accident will faithfully get recorded to your ~/.bash_history file when you logout, where another unscrupulous user might happen to find it. Editing your .bash_history manually won't fix the problem, as the file gets rewritten each time you log out.

To clear out your history quickly, try this from the command line:

export HISTSIZE=0

This completely clears out the current bash history and will write an empty .bash_history on logout. When you log back in, your history will start over from scratch but will otherwise work just as before. From now on, try to be more careful!

Do you have a problem with people logging into a machine, then disconnecting their laptop and going home without logging back out again? If you've ever run a w and seen a bunch of idle users who have been logged in for several days, try setting this in their environment:

export TMOUT=600

The TMOUT variable specifies the number of seconds that bash will wait for input on a command line before logging the user out automatically. This won't help if your users are sitting in a vi window but will alleviate the problem of users just sitting at an idle shell. Ten minutes might be a little short for some users, but kindly remind them that if they don't like the system default, they are free to reset the variable themselves.

This brings up an interesting point: exactly where do you go to make any of these environment changes permanent? There are several files that bash consults when starting up, depending on whether the shell was called at login or from within another shell.

From bash(1), on login shells:

...it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable . . . . When a login shell exits, bash reads and executes commands from the file ~/.bash_logout , if it exists.

For all other shells:

bash reads and executes commands from ~/.bashrc , if that file exists.

For the full definition of what constitutes a login shell (and for a whole bunch of information about the environment you work in every day), consult bash(1).

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