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Run Processes in the Background

Table of Contents

Background Processes with &

You can background any process by appending & to the end.
Using & to background a process: 

ping 127.0.0.1 &
This will direct stdout to the current shell, but the process will
run in the background.

This is useful if you want to run a process in the current
shell but also free it up to run other commands.

To stop the process, you can use fg to bring it back to the foreground, then ^C will stop it.

Alternatively, use ps to find its PID (Process ID), then use kill <PID>

Directing Output of Background Processes

See the bash hacker's redirection guide.

If you don't want to see the output of a background process in your current shell, you can redirect it: 

{ some_command & } > /dev/null 2>&1

  • This redirects both stdout and stderr to /dev/null (trash can).
  • It's used in a command group to allow the & syntax to be used. 
mkfifo /tmp/temp_pipe
{ some_command & } > /tmp/temp_pipe 2>&1
  • This redirects the output to a named pipe, which can be read by another process.

Backgrounding Subshells and Subshell Commands

You can background subshells, or commands within subshells, the same way as other processes with &.

Using & with Subshells (Inside vs. Outside)

  • Inside (command &): Only puts the commands within the subshell in the background.
  • Outside (command) &: Puts the entire subshell in the background.

Both approaches effectively run the commands in the background.

The placement of & inside or outside the subshell can influence how the background process is managed, and its relationship to the parent shell.
The distinction primarily affects how you think about the process grouping and management.

In terms of readability and understanding:

  • Placing & inside might suggest only the command is meant to run in the background.
  • Placing it outside clearly indicates the entire set of operations (the subshell) is intended to be backgrounded.

& Inside the Subshell (command &)

Scope of Background Process

  • When you place & inside the parentheses, it puts only the commands within the subshell in the background.
  • This is particularly useful if you want to group several commands together and run that group in the background.

Subshell Environment

  • The commands inside the parentheses are executed in a subshell.
    • This means they have a separate execution environment from the parent shell.
  • Variables modified in the subshell do not affect the parent shell.

Immediate Return

  • The parent script does not wait for the subshell to complete; it immediately continues to the next line of the script.

& Outside the Subshell (command) &

Entire Subshell in Background

  • Placing & outside the parentheses puts the entire subshell in the background.
  • This is similar to the above in practice but emphasizes that the subshell itself is being backgrounded.

Parent Script Continuation

  • Similar to the above, the parent script continues execution without waiting for the background process to complete.

Subshell Benefits

  • You still benefit from the isolated environment of the subshell, where commands executed inside do not directly modify the environment of the parent shell.

Using nohup

  • nohup - run a command immune to hangups, with output to a non-tty

This means that it will ignore the hangup signal, and will run in the background.

If the program that you want background is a script, you can use
the nohup command to run it in the background.