Shell Customization: Aliases, Functions and the Prompt

The environment variables lesson explained what .bashrc is. This lesson is about making the shell yours: shortcuts for commands you type fifty times a day, small functions that stitch commands together, a prompt that shows the information you actually need, and the startup-file rules that decide when each setting loads. A well-configured shell removes friction from every task that follows.

Aliases

An alias substitutes one word for a longer command. It is expanded before the rest of the line is parsed, so any extra arguments are appended.

bash

alias ll='ls -alF --color=auto'
alias la='ls -A'
alias ..='cd ..'
alias ...='cd ../..'
alias gs='git status'
alias gl='git log --oneline --graph --decorate -20'
alias ports='ss -tulpn'
alias please='sudo'
alias rm='rm -i'        # safety net
alias grep='grep --color=auto'

bash

alias            # list all aliases
alias ll         # show one
unalias ll       # remove
\ls              # run the real command, bypassing an alias
type ll          # "ll is aliased to `ls -alF --color=auto'"

Aliases are stored in memory only; put them in ~/.bashrc (or a separate ~/.bash_aliases that .bashrc sources) to keep them between sessions. Use single quotes so nothing is expanded at definition time.

Functions: When an Alias Is Not Enough

Aliases cannot use arguments in the middle of a command or contain logic. Shell functions can, and they live in the same file.

bash

# make a directory and enter it
mkcd() {
  mkdir -p "$1" && cd "$1"
}

# back up a file with a timestamp
bak() {
  cp -v "$1" "$1.$(date +%Y%m%d-%H%M%S).bak"
}

# search running processes by name
psg() {
  ps aux | grep -v grep | grep -i --color=auto "$1"
}

# universal extractor
extract() {
  case "$1" in
    *.tar.gz|*.tgz) tar xzf "$1" ;;
    *.tar.xz)       tar xJf "$1" ;;
    *.zip)          unzip "$1" ;;
    *.gz)           gunzip "$1" ;;
    *)              echo "extract: unknown archive type: $1" ;;
  esac
}

Inside a function $1, $2 and $@ are the arguments, exactly as in a script. Always quote them. A function defined in .bashrc is available in every interactive shell but not in scripts, which start clean.

Customizing the Prompt

The prompt is the value of PS1, re-evaluated before every line. Backslash escapes insert live information, and ANSI colour codes must be wrapped in \[ and \] so bash can measure the visible length correctly.

| Escape | Shows | |---|---| | \u | username | | \h | hostname (short) | | \w | current directory, with ~ for home | | \W | basename of the current directory | | \$ | $ for users, # for root | | \t | time, 24-hour | | \n | newline | | \[\e[32m\] … \[\e[0m\] | start green … reset colour |

bash

# Simple two-colour prompt
PS1='\[\e[32m\]\u@\h\[\e[0m\]:\[\e[34m\]\w\[\e[0m\]\$ '

# With the current git branch
parse_git_branch() {
  git branch 2>/dev/null | sed -n 's/^\* \(.*\)/ (\1)/p'
}
PS1='\[\e[32m\]\u@\h\[\e[0m\]:\[\e[34m\]\w\[\e[33m\]$(parse_git_branch)\[\e[0m\]\$ '

# Two-line prompt: path on the first line, a clean second line for typing
PS1='\[\e[34m\]\w\[\e[0m\]\n\$ '

The $(parse_git_branch) inside single quotes is evaluated every time the prompt is drawn, which is what you want. Colour codes: 31 red, 32 green, 33 yellow, 34 blue, 35 magenta, 36 cyan; 1;32 for bold green.

Which File Loads When

Bash reads different files depending on how it was started, and this is the most common reason a customization "does not work".

| Shell type | Files read | Examples | |---|---|---| | Login shell | /etc/profile, then the first of ~/.bash_profile, ~/.bash_login, ~/.profile | SSH session, text console | | Interactive non-login | /etc/bash.bashrc, ~/.bashrc | new terminal tab in a desktop | | Non-interactive | only $BASH_ENV if set | scripts, cron |

The convention that makes everything work: put aliases, functions and PS1 in ~/.bashrc, and make sure ~/.profile (or ~/.bash_profile) sources it. Ubuntu's default ~/.profile already does. Environment variables such as PATH belong in ~/.profile so login shells and graphical sessions inherit them.

bash

# ~/.bash_profile on systems that do not have this yet
[ -f ~/.bashrc ] && . ~/.bashrc

After editing, run source ~/.bashrc (or . ~/.bashrc) to apply changes in the current shell.

Useful Shell Options

bash

shopt -s autocd          # type a directory name to cd into it
shopt -s cdspell         # correct minor typos in cd
shopt -s histappend      # append history rather than overwrite on exit
shopt -s globstar        # ** matches recursively: ls **/*.js
HISTSIZE=50000
HISTFILESIZE=100000
HISTCONTROL=ignoreboth   # skip duplicates and lines starting with a space
export EDITOR=vim
set -o vi                # vi-style line editing, if you prefer it

Common Mistakes

Defining an alias with double quotes and a $(...), which runs once at definition time instead of each use.
Naming a function the same as a command you still need (ls() {...}), then losing access to the original. Use command ls inside the function.
Forgetting \[ \] around colour codes, which makes line editing jump and wrap incorrectly.
Putting PATH changes only in .bashrc, so GUI applications and cron do not see them.

Key Takeaways

Aliases are text substitutions for frequent commands; define them in ~/.bashrc with single quotes.
Functions accept arguments and logic; quote $1 and $@, and use command to call the original program.
PS1 builds the prompt from escapes like \u, \h, \w; wrap colour codes in \[ \].
Login shells read ~/.profile; interactive shells read ~/.bashrc; have the former source the latter.
source ~/.bashrc applies edits immediately; shopt and HIST* variables tune behaviour further.

Next lesson: Shell Scripting Basics — turn sequences of commands into reusable scripts with a shebang, variables and simple control flow.