Environment & Startup Files

Startup Files

Which configuration files Bash reads depends on how it was started. Getting this wrong is the classic "it works in my terminal but not in cron" problem.

Bash classifies itself first, then reads the matching startup file.

A login shell is one you authenticate into: an SSH session, a console login, or bash -l / bash --login. It reads:

/etc/profile — system-wide settings
the first existing of ~/.bash_profile, ~/.bash_login, ~/.profile

Interactive Non-Login Shell

Opening a new tab or terminal window after logging in usually gives an interactive non-login shell. It reads /etc/bash.bashrc (on Debian/Ubuntu) and ~/.bashrc.

Non-Interactive (Scripts)

A script is non-interactive. It reads none of the above. It only honours the file named by $BASH_ENV, which is normally unset. This is why aliases and interactive functions are invisible to your scripts.

bash -l            # force a login shell
bash -i            # force an interactive shell
bash --noprofile --norc -c 'echo hi'   # skip all startup files (useful for tests)

profile vs bashrc

Put environment settings in ~/.profile/~/.bash_profile (PATH, EDITOR, locale) and interactive settings in ~/.bashrc (aliases, prompt, completion). Then let one source the other so a login shell gets both.

# ~/.bash_profile
[[ -f ~/.bashrc ]] && source ~/.bashrc
export PATH="$HOME/.local/bin:$PATH"

# ~/.bashrc
export PS1='\[\e[32m\]\u@\h\[\e[0m\]:\w\$ '
alias ll='ls -alF'
alias gs='git status'
shopt -s checkwinsize histappend

After editing a startup file, run source ~/.bashrc to apply it to the current shell, or open a new terminal.

Environment Variables

Common Variables

Variable	Purpose
`PATH`	Directories searched for commands
`HOME`	Your home directory
`USER`	Current username
`PWD` / `OLDPWD`	Current and previous directory
`SHELL`	Login shell path
`LANG` / `LC_*`	Locale and character set
`EDITOR`	Preferred text editor

Shell-Maintained Variables

echo "$BASH_VERSION"     # 5.2.21(1)-release
echo "$BASH_SOURCE"      # current sourced file path
echo "$LINENO"           # current line number (useful in errors)
echo "$SECONDS"          # seconds since the shell started
echo "$RANDOM"           # pseudo-random integer 0..32767
echo "$PWD"              # logical current directory

$RANDOM is fine for games and temp names but is not cryptographically secure. Use openssl rand or /dev/urandom for secrets.

Shell Options

set Options

set -e            # exit on any command failure
set -u            # error on unset variables
set -o pipefail   # a pipeline fails if any stage fails
set -x            # print commands as they run (debug)
set +x            # stop tracing

shopt Options

shopt -s nullglob      # unmatched globs expand to nothing
shopt -s globstar      # '**' matches across directories
shopt -s nocaseglob    # case-insensitive globbing
shopt -s extglob       # extended pattern operators
shopt -u nullglob      # turn an option off

Pitfalls

Scripts never read ~/.bashrc: do not depend on aliases inside scripts.
PATH in cron is minimal — set it explicitly or use absolute paths.
Exporting a variable affects only children; a child cannot change the parent's environment.
Editing ~/.bashrc can break every new shell — test with bash --rcfile before committing.