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Shells

What is a shell?

A [Li|U]nix shell is the command-line interface between the user and the system. It is used to perform some action, specifically, typing commands and displaying output, requested by the user.

Introduction to Bash

Bash is known as the Bourne-again shell, written by Brian Fox, and is a play on the name of the previously named Bourne shell (/bin/sh) written by Steve Bourne. [1]

Shell fundamentals

Command-line Editing Modes

  • vi

    set -o vi

  • emacs

    set -o emacs

See :doc:`text_editing_101` for details on appropriate edit commands to use on the command line.

Environment variables

Environment variables are used to define values for often-used attributes of a user's shell. In total, these variables define the user's environment. Some environment variables provide a simple value describing some basic attribute, such the user's current directory ($PWD). Others define the behavior of a command, such as whether or not the history command should log repeated commands individually or log the repeated command once ($HISTCONTROL).

$PATH

The most common, or most recognized, environment variable is the $PATH variable. It defines the set of directories that the shell can search to find a command. Without an explicit path provided when calling a command (i.e. /bin/ps), the shell will search the directories listed in the $PATH variable until it finds the command. If the command is not found in any of the defined directories in $PATH, the shell will produce an error explaining as much.

$ foobar -V
-bash: foobar: command not found

To view the contents of the $PATH variable, use echo to print the variable's value:

$ echo $PATH
/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/bin

The order of the directories in the $PATH variable, from left to right, is important; when searching directories for a command, the shell will stop looking after it finds its first match. In other words, using our example $PATH variable above, if there is a version of ps that exists in /usr/local/bin that is preferred (by the sysadmin) over the version that exists in /bin, the shell will still execute /bin/ps due to the precedence of the directories defined in the $PATH variable.

To list all of the shell's environment variables, use the env command:

$ env
HOSTNAME=foobar
SHELL=/bin/bash
TERM=xterm
HISTSIZE=1000
USER=root
PATH=/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/bin
MAIL=/var/spool/mail/root
PWD=/root/curriculum
PS1=[\[\e[33;1m\]\t \[\e[31;1m\]\u\[\e[0m\]@\[\e[31;1m\]\h\[\e[0m\] \W\[\e[0m\]]#
AWS_IAM_HOME=/opt/aws/apitools/iam
HISTCONTROL=ignoredups
SHLVL=1
SUDO_COMMAND=/bin/bash
HOME=/root
HISTTIMEFORMAT=[%Y-%m-%d %H:%M:%S]
OLDPWD=/tmp

Global vs. User Profiles

  • /etc/profile, ~/.bash_profile
  • Profile precedence

Special environment variables

$$, $!, $?, etc

History

history, .bash_history, !!, !# (i.e. !1 to repeat the first command in the history), ^R, etc

Job control

  • ^Z, bg, fg, %1/2/3.., jobs

For information on ensuring running jobs continue, even when terminal connectivity is lost, see the sections on :ref:`gnu-screen` and :ref:`tmux`.

Customizing the Prompt for Fun or Profit

$PS1, $PS2, $PS3, $PS4

Example (needs explanation):

# if I'm root, set my terminal colors to alert me!
if [ "$EUID" = "0" ]
then
  PS1="[\[\e[33;1m\]\t \[\e[31;1m\]\u\[\e[0m\]@\[\e[31;1m\]\h\[\e[0m\] \W\[\e[0m\]]\$ "
  export PS1
else
  PS1="[\t \[\e[34;1m\]\u\[\e[0m\]@\[\e[34;1m\]\h\[\e[0m\] \[\e[33;1m\]\W\[\e[0m\]]\$ "
  export PS1
fi

.. todo::
   - Link to any content describing profiles (global, user-level) as the above example should be placed in a profile
   - Link to content describing terminal color codes/ANSI escape codes
   - Determine if it's important to discuss such an esoteric topic as terminal color/escape codes or if I'm really just showing off...

Footnotes

[1]C Programming by Al Stevens, Dr. Dobb's Journal, July 1, 2001