How to enable command completion in Bash

Command completion in Bash lets the shell offer command-aware matches when Tab is pressed, including subcommands, options, and arguments provided by installed completion scripts. Basic filename completion can work without extra packages, but command-specific completion needs Bash's programmable completion layer and a completion loader.

On Ubuntu and Debian systems, the bash-completion package installs a shared completion loader and the /etc/bash_completion compatibility entry point. Current default Ubuntu startup files already source one of those paths for interactive shells, but older or custom ~/.bashrc files may need the loader added manually.

The package is enabled when an interactive Bash shell can find _comp_complete_load, the on-demand loader function. If that check already prints the function name after reloading ~/.bashrc, the completion loader is active and no startup-file edit is needed.

Steps to enable command completion in Bash:

Confirm that programmable completion is enabled in the current Bash shell.
```
$ shopt progcomp
progcomp        on
```
If this returns off, run shopt -s progcomp in the interactive shell or remove the startup-file line that disables progcomp.
Install the bash-completion package on Ubuntu or Debian.
```
$ sudo apt install bash-completion
```
The package manager prints the normal install summary and finishes by setting up the bash-completion package.
Confirm that the package installed the Debian and Ubuntu compatibility loader.
```
$ ls /etc/bash_completion
/etc/bash_completion
```
Use the existing distro snippet if ~/.bashrc already has one. The short manual snippet below is for custom startup files that do not load bash-completion yet.
Back up ~/.bashrc before changing interactive shell startup behavior.
```
$ cp ~/.bashrc ~/.bashrc~
```
If ~/.bashrc does not exist yet, create it with touch ~/.bashrc before checking or adding the completion loader.
Reload ~/.bashrc and check whether the loader function is already present.
```
$ source ~/.bashrc
$ f=_comp_complete_load
$ declare -F "$f"
_comp_complete_load
```
If this command prints _comp_complete_load, skip the ~/.bashrc edit and open a new terminal before using command-specific completion.
Add the bash-completion loader to ~/.bashrc only when the previous check printed no handler.
~/.bashrc
```
. /etc/bash_completion
```
Reload ~/.bashrc in the current terminal.
```
$ source ~/.bashrc
```
Confirm that the interactive shell can find the completion loader function.
```
$ f=_comp_complete_load
$ declare -F "$f"
_comp_complete_load
```
The _comp_complete_load function is the bash-completion on-demand loader. It loads command-specific completion files when completion is requested for a command.