Explain .bashrc code

What this .bashrc sample shows

interactive shells

bash reads ~/.bashrc each time it starts an interactive shell, the kind you type into. $- holds the shell's options and has an i only when it's interactive, so a script that ran source ~/.bashrc stopped at that line: its next line still ran, but type ll said ll: not found.

variables and PATH

export hands a variable on to every program the shell starts, while HISTSIZE and HISTCONTROL are settings for bash itself. PATH is the list of folders searched for a command, in order, so with $HOME/bin put first, command -v hello found the hello script in ~/bin.

aliases and functions

An alias swaps a short word for a longer command: type ll reported that ll is aliased to ls -lah. A function can take arguments, which an alias can't: mkcd a/b made both folders and moved into them, so pwd printed ~/a/b.

the prompt

PS1 is the prompt. \u, \h and \w stand for your user name, the computer's name and the current folder, and $(...) runs a command each time the prompt is drawn. In a repo on the feature/login branch, the prompt ended in ~/proj (feature/login)$.

Good to know

• Run with GNU bash 5.3.15 on a test home folder, so nothing touched a real ~/.bashrc. Changes reach new terminals; in an open one, source ~/.bashrc reads the file again.
• Aliases are for typing, not for scripts: a script that defined alias ll="ls -lah" and then ran ll stopped with ll: command not found. Write a function when a script needs the shortcut.
• The bash manual on the history lines: erasedups "causes all previous lines matching the current line to be removed from the history list before that line is saved", and with histappend "the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file."

Watch out: A login shell doesn't read ~/.bashrc. Started as bash -l, type ll printed ll: not found, because a login shell reads ~/.bash_profile instead. With [ -f ~/.bashrc ] && . ~/.bashrc in ~/.bash_profile, the same login shell had the alias.