Frequently Asked Question

How does tab completion work?

Pressing Tab asks the shell to finish what you've started typing. With no extra configuration, bash completes command names by searching PATH, and file and directory names by listing what's in the relevant directory. cd ~/Doc<Tab> expands to cd ~/Documents/; git che<Tab> expands to git checkout if git is the only matching command.

Beyond the defaults, bash has a programmable completion system: the bash-completion package, shipped with most distributions, registers completion functions for hundreds of commands. With it installed, git che<Tab> completes sub-commands, systemctl restart ngi<Tab> completes unit names, apt install libssl<Tab> completes package names. Internally each completion is a shell function registered with the complete builtin; you can write your own with complete -F mycomplete mycommand. Zsh and fish ship with much more aggressive completion out of the box, fish in particular reads completions straight from man pages.