Bash - (Simple) (Command | Expression)

Process - Command (Command line) in bash are also 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.

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

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.

See Bash - Command Execution

# 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; }
}

Linux - man (Manual, Help, Documentation)

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

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 <note warning>and not the command 1006 because it does not take into account the option !</note>

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

• A Comparison of Common DOS and Linux Commands