Getting Help and Typing Less

Four kinds of commands

A command can be one of four things ¹:

1. A program: a file on disk, like /usr/bin/ls. Most commands are these.
2. A shell builtin: part of bash itself, like cd, echo, and type.
3. A shell function: a little script loaded into the shell.
4. An alias: a nickname you or your distribution defined for another command.

type tells you which ¹:

me@linuxbox:~$ type type
type is a shell builtin
me@linuxbox:~$ type ls
ls is aliased to `ls --color=auto'
me@linuxbox:~$ type cp
cp is /usr/bin/cp

which finds only programs: it searches the folders listed in $PATH and prints the first match ^{1 2}. That’s why which cd comes back empty: cd isn’t a file, it’s built into the shell ². The kind matters for help, because builtins and programs document themselves differently.

Asking for help

• help documents bash’s builtins: help cd ¹.
• --help makes most programs print a usage summary: mkdir --help. Not every program supports it, but trying does no harm ¹.
• man shows the manual page: man ls ^{1 2}.
• tldr shows a handful of common examples instead of every option ². Ubuntu packages it: sudo apt install tldr, then try tldr tar. (If it says it has no pages yet, run tldr --update once.)

When you’re stuck, tldr for the common case, --help for a quick reminder, man for the full truth.

Man pages

man displays the page in a pager called less, so these keys work ¹:

A man page gives the name, a synopsis of the syntax, a description, and every option. It’s a reference, not a tutorial, so pages rarely include examples ¹. In the synopsis, things in [brackets] are optional and ... means “can repeat.”

The manual is divided into sections ¹. You’ll mostly use:

• 1, user commands (ls, grep)
• 5, file formats (/etc/passwd)
• 8, system administration commands (mount)

When a name exists in more than one section, give the number: man passwd shows the command, man 5 passwd the format of the file.

Two ways to search the manual itself ¹:

me@linuxbox:~$ whatis ls
ls (1)               - list directory contents
me@linuxbox:~$ apropos partition

whatis prints a page’s one-line description. apropos (same as man -k) lists every page whose description matches a word, which is how you find a command you don’t know the name of yet.

Tab completion

Press Tab while typing a command or a file name and bash finishes it, as far as it can without guessing ^{1 2}. If several names match, press Tab twice to list them, type another letter or two, and Tab again.

me@linuxbox:~$ cd Doc<Tab>
me@linuxbox:~$ cd Documents/

Tab completion is the single biggest speed-up, and it prevents typos: if Tab won’t complete a path, the path you’re typing doesn’t exist.

History

Bash keeps your past commands, the last 1,000 on most distributions ¹:

• Up / Down arrows step through them.
• history lists them with numbers. history | grep rsync finds old ones (| is a pipe, coming in module 4).
• Ctrl-R searches backward as you type: press it, type rsync, and the most recent matching command appears. Ctrl-R again for older matches, Enter to run it, or an arrow key to edit it first ¹.
• !! repeats the last command. The classic use: sudo !! after a command fails with “Permission denied” ¹.

Moving and editing on the line

These keys come from Readline, the line-editing library bash uses, and many other command-line programs honor them too ¹:

• Ctrl-A / Ctrl-E: jump to the start / end of the line.
• Alt-F / Alt-B: forward / back one word.
• Ctrl-U / Ctrl-K: cut from the cursor to the start / end of the line.
• Alt-Backspace: cut the word before the cursor.
• Ctrl-Y: paste back (“yank”) what you cut.
• Ctrl-L: clear the screen (same as clear).

If Alt doesn’t work in your terminal, press and release Esc first, then the letter ¹.

Your turn

So

Ask the shell what a command is with type, ask the command how to use it with --help, man, or tldr, and let Tab and history do your typing. These habits make every later lesson faster.