Customising the shell

As noted in previous sections, the shell can be configured in various way to make it easier to use or more powerful. Many of the more useful modifications can be made via the readline library, since this is the part of the shell that allows commands to be edited. There are also various environment variables that can be set, for instance to modify the prompt, or store more commands in the history.

Startup files

All of these modifications are typically implemented by adding the relevant commands to the shell’s various startup files. Part of the difficulty in getting these modifications to work lie in figuring which file gets executed under what circumstances, and what settings this file affects.

When the shell starts, it will typically be invoked as a login or non-login shell, depending on whether you have just logged into the system directly into this shell. Generally, when you log into a desktop session, the session will be started from the shell, and this shell is the login shell. This means any terminal sessions you start subsequently from this graphical session are non-login shells (although not on macOs, by default). The reason this matters is because different startup files are used depending on whether it’s a login shell or not. This means that settings placed in one startup file (e.g. ~/.bashrc) may have no effect when logging in via a remote SSH connection, for instance, since this session would start a login shell. On top of that, different distributions have come up with subtly different conventions as to what files are used, making it very difficult to make recommendations guaranteed to work in all cases.

The information below relates to the Bourne Again shell (BASH), and will hopefully be representative of most systems (see the official BASH documentation for full details). The relevant files would be different for different shells:

  • login shells will typically read the system-wide /etc/profile if it exists, then the user-specific ~/.profile or ~/.bash_profile if it exists.
  • non-login shells will typically read the ~/.bashrc file if it exists.
  • on many distributions, the system-wide /etc/profile will contain instructions to read the system-wde /etc/bash.bashrc file if it exists.
  • likewise, on many distributions, the user ~/.profile will contain instructions to run the user ~/.bashrc file if found.
  • Settings that affect the readline library will normally go in the ~/.inputrc file (see below).

As you can appreciate, things can rapidly become complicated. In most cases, you should be able to add your settings to the ~/.bashrc file, and on a properly configured system, that should be sufficient.

Useful ~/.bashrc customisations

These suggestions are things that I’ve found useful to add to ~/.bashrc, you may find some of them to your taste:

  • Append to the history, do not overwrite it:

    shopt -s histappend
  • Keep a lot more commands in the history than the default:

    export HISTSIZE=10000 HISTFILESIZE=100000
  • Don’t keep duplicate entries in the command history:

    export HISTCONTROL=ignoreboth
  • On colour-capable terminals, use colours in file listings:

    alias ls='ls --color=auto'
  • Make common operations prompt you if they are about to overwrite files:

    alias rm='rm -i'
    alias cp='cp -i'
    alias mv='mv -i'

Useful readline customisations

Most distributions will come with the following already set, but just in case, the following is useful to put into your ~/.inputrc if your Home, End, or Del keys don’t work, and if you want to be able to skip over words with Ctrl+Left/Right:

# mappings for Home/End:
"\e[1~": beginning-of-line
"\e[4~": end-of-line
"\e[7~": beginning-of-line

# Del key:
"\e[3~": delete-char

# Ctrl+arrows to skip words:
"\e[5C": forward-word
"\e[5D": backward-word
"\e\e[C": forward-word
"\e\e[D": backward-word
"\e[1;5C": forward-word
"\e[1;5D": backward-word

Generally, most distribution set up the Up & Down arrows to go through the history, with PgUp & PgDn going to the oldest and most recent entry respectively. I find a more useful use for the Up & Down arrows is to perform a search through the history. If nothing has been typed yet, this just goes through the history as is normally the case. But as soon as a few characters have been entered, only those commands in the history that start with the same fragment will come up when you press Up. This allows you to quickly retrieve a command you might have typed quite some time ago, as long as you know how it started:

# alternate mappings for "up" and "down" to search the history
"\e[A": history-search-backward
"\e[B": history-search-forward

For example, if you set a complicated environment variable at the beginning of your session, but now need to modify its value slightly, you could just type exp followed by the Up arrow key, and the chances are the first match will be the export COMPLICATED_VARIABLE=some_other_complicated_value line that you wanted to edit – no need to type it all in again…