Tab completion, history and editing the command line · Lesson 8

Nobody types full paths. Typing /root/lab/l8/a-very-long-directory-name/ by hand is slow and error-prone, and the shell already knows what is on disk. Press Tab after the first few letters and it finishes the name for you. This is not a convenience; it is how the command line is meant to be used, and it doubles as a spell checker: if Tab does nothing, the thing you are typing does not exist.

How Tab decides

The shell looks at the word under the cursor. If it is the first word of the line, it completes command names: mkd plus Tab becomes mkdir. Anywhere else it completes file and directory names relative to what you have typed so far. If exactly one name matches, it is filled in — with a trailing / for a directory, so you can keep going deeper. If several match, Tab fills in the common part and stops; press Tab a second time to see the list of candidates, type one more letter to disambiguate, and Tab again.

~% ls lab/l8/
a-very-long-directory-name-nobody-wants-to-type  report-2024-final-version-2.txt
~% cd lab/l8/a<Tab>
~% cd lab/l8/a-very-long-directory-name-nobody-wants-to-type/
~% ls lab/l8/r<Tab>
~% ls lab/l8/report-2024-final-version-2.txt

A rhythm develops: a few letters, Tab, a few letters, Tab. If Tab beeps or does nothing, stop and check your spelling or your position with ls — do not keep typing. The practice machine's shell completes names but not options; bash on a desktop can do options too, with extra packages.

History

Every line you run is remembered. The Up arrow brings back the previous one, Up again the one before, Down goes forward. The recalled line is editable: fix the typo, press Enter, and the corrected command runs. history prints the list with numbers. The list is saved to ~/.ash_history (.bash_history under bash) when the shell exits, so on a normal system it survives logouts; on this temporary machine it vanishes with everything else.

Moving inside the line

Keys	Effect
Tab	Complete the current word; twice to list candidates.
Up / Down	Previous / next command from history.
Ctrl-A / Ctrl-E	Jump to the start / end of the line.
Ctrl-W	Delete the word before the cursor.
Ctrl-U	Delete everything before the cursor: a clean slate.
Ctrl-K	Delete from the cursor to the end.
Ctrl-C	Abandon the line (or interrupt a running command) and get a fresh prompt.
Ctrl-L	Clear the screen, keeping the current line.

These are Emacs-style bindings and they work in bash, zsh, BusyBox ash, Python's prompt, psql and most other line-oriented programs — learning them once pays everywhere. The most valuable single habit is Ctrl-C to bail out: whenever a line has gone wrong, or a command is taking too long, Ctrl-C returns you to a clean prompt.

Bash's !! (repeat the last command) and !123 (run history entry 123) are history expansion, a bash feature. BusyBox ash does not have it; use the Up arrow instead, which works in both.

Commands in this lesson

Command	What it does
`<Tab>`	Complete the word under the cursor.
`<Up> / <Down>`	Walk through previous commands.
`history`	Print the numbered list of past commands.
`Ctrl-A / Ctrl-E`	Start / end of line.
`Ctrl-W / Ctrl-U`	Delete the previous word / the whole line before the cursor.
`Ctrl-C`	Abandon the current line or interrupt the running command.

Quiz

You type `cd /ro` and press Tab. What happens?

The shell completes it to `cd /root/`
The shell runs `cd /ro` and fails
Nothing; Tab only completes command names

Tab fills in part of a name and stops. What should you do?

Press Tab again to see the candidates, then type a letter to disambiguate
Press Enter; the shell will pick one
Delete the line and start over

Which key combination abandons the line you are typing and gives a fresh prompt?

Ctrl-A
Ctrl-C
Ctrl-E

What does `history` print?

The files you edited recently
A numbered list of the commands you have run
The system's boot log

Why does `!!` not work on the practice machine?

History is disabled
History expansion is a bash feature that ash lacks; use the Up arrow
Root is not allowed to repeat commands

Practice

Print the list of commands you have run so far.

Using Tab to complete the name, create an empty file called `done.txt` inside /root/lab/l8/a-very-long-directory-name-nobody-wants-to-type.

/root/lab/l8 contains `report-2024-final-version-2.txt`. Create a sibling file with the same name but the extension `.bak` — let Tab type the long name, then edit the end of the line.

Open this lesson in the app to do the tasks in a real Linux machine and have them checked.