Startup Files and the Prompt

Two kinds of shell

When bash starts, it reads startup files: little scripts that set up your environment. Which ones depends on the kind of shell ^{1 2}:

• A login shell is one you logged in to: a text console, an SSH session, or the shell WSL opens for you. It reads /etc/profile, then the first one it finds of ~/.bash_profile, ~/.bash_login, and ~/.profile ².
• An interactive non-login shell is one started inside a session you’re already logged in to, like a new terminal window on a Linux desktop. It reads /etc/bash.bashrc and ~/.bashrc, and inherits environment variables from the session that started it ^{1 2}.

Files under /etc apply to everyone; files in your home are yours to edit. Ubuntu gives each user a ~/.profile (no .bash_profile), and that .profile reads ~/.bashrc, so in practice both kinds of shell end up reading ~/.bashrc ¹.

What goes where

• ~/.profile: environment variables, which every program inherits. PATH additions and export EDITOR=nano belong here ¹.
• ~/.bashrc: things for interactive use: aliases, functions, the prompt, history settings, shell options ¹.

The reason for the split: environment variables need setting once per login and are inherited from there, while aliases and functions aren’t inherited, so every new shell has to read them.

Back up a startup file before editing it: cp ~/.bashrc ~/.bashrc.orig. A mistake in it shows up as errors every time a terminal opens, and the backup gets you out.

Applying changes

Startup files run when a shell starts. After editing one, either open a new terminal or load it into the current shell with source ¹:

me@linuxbox:~$ nano ~/.bashrc
me@linuxbox:~$ source ~/.bashrc         # or the short form: . ~/.bashrc

source runs a file’s commands in your current shell, rather than in a new one, which is why it can change your aliases and variables.

A tour of Ubuntu’s ~/.bashrc

Open yours with less ~/.bashrc. Highlights of Ubuntu’s default:

HISTCONTROL=ignoreboth      # don't save duplicates or lines starting with a space
shopt -s histappend         # add to the history file instead of overwriting it
HISTSIZE=1000               # commands kept in memory
HISTFILESIZE=2000           # commands kept in ~/.bash_history
shopt -s checkwinsize       # keep line wrapping right when the window resizes

alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'

if [ -f ~/.bash_aliases ]; then
    . ~/.bash_aliases
fi

The last part matters: Ubuntu’s .bashrc reads ~/.bash_aliases if it exists. That’s the tidy place for your own aliases and functions; create the file and leave .bashrc itself alone. A leading space keeps a command out of history (that’s what ignoreboth includes), handy for commands with a password in them. HISTSIZE=10000 is a common, worthwhile change ².

Your own prompt

The prompt is the variable PS1. Backslash codes in it are replaced each time the prompt is drawn ¹:

Try one out in the current shell (it lasts until you close it) ¹:

me@linuxbox:~$ PS1='\A \w\$ '
09:41 ~$ cd /etc
09:41 /etc$

Colors are escape sequences such as \e[1;32m (bright green) and \e[0m (back to normal). Wrap each in \[ and \] so bash knows they take no space on screen; otherwise long lines wrap wrongly ¹:

PS1='\[\e[1;32m\]\u@\h\[\e[0m\]:\[\e[1;34m\]\w\[\e[0m\]\$ '

That’s roughly Ubuntu’s default: a green user@host, a blue directory. When you’ve got one you like, add the PS1= line to the end of ~/.bashrc.

Your turn

So

Login shells read /etc/profile and ~/.profile; interactive shells read ~/.bashrc, and on Ubuntu .profile reads .bashrc too. Put environment variables and PATH in ~/.profile, and aliases, functions, history settings, and the prompt in ~/.bashrc (or ~/.bash_aliases), then source the file to apply it.