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Debian GNU/Linux Reference Guide
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4.3.10 Typical command sequences

Let's try to remember following shell command idioms. See Shell parameters, Section 13.2.3, Shell redirection, Section 13.2.4, Shell conditionals, Section 13.2.5, and Command-line processing, Section 13.2.6 after reading these idioms.


4.3.10.1 command &

The command is executed in the subshell in the background. Background jobs allow users to run multiple programs in a single shell.

The management of the background process involves the shell built-ins: jobs, fg, bg, and kill. Please read the sections of the bash(1) manual page under "SIGNALS", "JOB CONTROL", and "SHELL BUILTIN COMMANDS". [ 29]


4.3.10.2 command1 | command2

The standard output of command1 is fed to the standard input of command2 . Both commands may be running concurrently. This is called pipeline.


4.3.10.3 command1 ; command2

The command1 and command2 are executed sequentially.


4.3.10.4 command1 && command2

The command1 is executed. If successful, command2 is also executed sequentially. Return success if both command1 and command2 are successful.


4.3.10.5 command1 || command2

The command1 is executed. If not successful, command2 is also executed sequentially. Return success if command1 or command2 are successful.


4.3.10.6 command > foo

Redirect standard output of command to a file foo. (overwrite)


4.3.10.7 command >> foo

Redirect standard output of command to a file foo. (append)


4.3.10.8 command > foo 2>&1

Redirect both standard output and standard error of command to a file foo.


4.3.10.9 command < foo

Redirect standard input of command to a file foo. Try:

     $ </etc/motd pager
      ... (the greetings)
     $ pager </etc/motd
      ... (the greetings)
     $ pager /etc/motd
      ... (the greetings)
     $ cat /etc/motd | pager
      ... (the greetings)

Although all 4 syntaxes display the same thing, the last example runs extra cat command and wastes resources with no reason.


Debian GNU/Linux Reference Guide
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