What is the Ampersand (&) in the Bash Shell ? ie the Asynchronous, Parallel control operator

About

The Ampersand (&) is:

• control operator
• that separates command in a list

and the shell:

• executes the command in the background in a subshell.
• does not wait for the command to finish,
• returns an return status of 0 (true, successful).

Example

The format for running commands in the background is:

command1 & [command2 & ...]

• With bash script

startdemo.sh&

How does it work ?

• By adding the ampersand (&) after a command, you start the application in background.
• When you use the command prompt to run the appropriate .sh files, by adding “&” you start them as a background task.

Where is the output redirected?

When job control is not active (see Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Process

The process inherits stdout/stderr from the shell (so it still writes to the terminal).

• The process in principle also inherits stdin, but as soon as it tries to read from stdin, it is halted.
• It is put into the list of background jobs the shell manages, which means especially:
  • It is listed with jobs and can be accessed using %n (where n is the job number).
  • It can be turned into a foreground job using fg, in which case it continues as if you would not have used & on it (and if it was stopped due to trying to read from standard input, it now can proceed to read from the terminal).

If the shell received a SIGHUP, it also sends a SIGHUP to the process. Depending on the shell and possibly on options set for the shell, when terminating the shell it will also send a SIGHUP to the process.