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This short article introduces the nohup command and &the control operator to run Linux processes in the background through Bash. In addition, we will further study &the key differences between nohup and .

Running Linux processes in the background

Linux provides two ways to run processes or commands in the background.

1. Use only the ampersand ( &) control operator.
2. Use the nohup command &with .

The & symbol (&) control operator

We can use the & control operator to run any command asynchronously in the background.

Consider the following command.

sleep 10

sleepcommand to add a delay of a specific time. When we run the sleep 10 command, it will pause the Bash terminal for 10 seconds and we cannot run any other command on the terminal.

Now, consider the following command.

sleep 10 &

The above command displays the process id (PID) and sleep 10 is executed asynchronously in the background. The execution control returns to the command terminal without waiting for the sleep to end.

Now, we can run any other command concurrently with the background sleep command on the same terminal.

We can move the background process to foreground using the following command.

fg

nohup Command

nohupcommand to run any other command or process. It stands for "no hang-up" and it prevents the associated process from getting the SIGHUP signal.

If you want the command to execute even after the terminal is closed, you can use nohup CommandName.

However, if we want to run a command in the background and the execution control returns to the terminal immediately, we have to use the following command.

nohup sleep 10 &

The above command runs the sleep 10 command in the background and returns the control immediately so that we can run any other command on the same terminal.

We can use the pgrep command to view the commands running in the background, as follows:

pgrep -a [Command]

pgrepCommand Search for a command and display the process ID (PID) and details of the command being executed.

For example, pgrep -a sleepthe relevant background processes are displayed as:

PID sleep 10

Here, PID represents the process ID assigned to the sleep command.

Difference between the control operator & and the nohup command

Following are some of the differences between using &and to run a command or process in the background.nohup

nohup can catch the hangup signal (SIGHUP), while & cannot. The SIGHUP signal is used to send a signal to a process when the terminal in which the process was started is closed.

Typically, a process or command is run in the background using & until there is a shell from which the command or process was launched. Once the shell terminates, all associated commands or processes running in the background with & will also terminate.

When the terminal exits, SIGHUP (kill SIGHUP <pid>)the hangup signal used will terminate all subcommands or subprocesses of the terminal. However, this can be prevented using nohup.

The nohup command captures the SIGHUP signal and does not let it reach the actual command, thus limiting the command to terminate when the Bash terminal exits.

&Another difference between and nohupis regarding the redirection of stdout/stderr.

&The operator does not automatically redirect stdout/stderr, but displays the command's output directly to the terminal. However, nohup redirects stdout/stderr to the file nohup.out located in $HOME.