What is a Command in Bash?

About

This page is about command in bash.

Command are in bash the expression unit

A command (or an expression) is a sequence of words separated by blanks, terminated by a control operator.

Each command execution has an environment.

Search Order

The default order for command lookup is:

• functions,
• built-ins (If you want to disable a builtin command, see the command Bash - Enable, disable or print Builtin command),
• Bash - Disk Command (ie with scripts and executables last)

There are three built-ins that you can use to override this order:

• Bash - Command (The Builtin command command) - Removes alias and function lookup. Only built-ins and commands found in the search path are executed
• Bash - Builtin Commands - Looks up only built-in commands, ignoring functions and commands found in PATH
• and Bash - Enable, disable or print Builtin command - Enables and disables shell built-ins

Exit Status

The return status (of Exit Status) is:

• its exit status as provided by the POSIX 1003.1 waitpid function,
• or 128+n if the command was terminated by signal n.

Management

Builder

Thanks to array parameter expansion, you can build a command with an array

Example building the command ls -a

cmd=("ls")
cmd+=("-a")
# execution
"${cmd[@]}"

Execution

See Bash - Command Execution

Check availibilty

# Check the command $1
function check_command() {
	command -v $1 -v 2>&1 > /dev/null || { echo >&2 "I require $1 but it's not installed.  Aborting."; exit 1; }
}

Help

Linux - man (Manual, Help, Documentation)

Alias

To create a shortcut of a command with parameters, see Bash - Alias (of a command) - Builtin command

List the history of the command called

history

With the command history:

$history

you will get an history of the command

1000  cat installation_overview_di.txt
 1001  fc -l 999
 1002  man fc
 1003  history
 1004  history|grep NQS
 1005  ls -al
 1006  ls -al
 1007  dir
 1008  history

The exclamation mark start a history command substitution, except when followed by a space character, tab, newline, = or (.

• !! execute the last command executed. It would execute the history command (1008)
• !str - Re runs the last command that you ran that starts with str (string). For instance, !ls would execute the last ls command ie the command 1006
• !1007 Rerun the command 1007. A dir command in our case.
• !-2 Refer to the current command line minus 2. Ie the command 1007 (dir)
• !?str? - Refer to the most recent command containing str (string). For instance, !?al? will start the command 1000 and not the command 1006 because it does not take into account the option !

fc

The 'fc' utility lists or edits and re-executes, commands previously entered to an interactive sh.

fc -l 999

will list the last 999 command

Ctrl + r

List of common command

Documentation / Reference

• A Comparison of Common DOS and Linux Commands