Shulou(Shulou.com)06/01 Report--

What is the specific use of Linux bash wait command, I believe that many inexperienced people do not know what to do, so this paper summarizes the causes of the problem and solutions, through this article I hope you can solve this problem.

The wait command suspends the current shell process and waits for the specified child process generated by the current shell to exit before the wait command returns. The argument to the wait command can be process ID or job.

Bash wait command

The general syntax built into wait takes the following form:

Wait [options] IDCopy

ID is a process or task ID. If no ID is specified, the command waits for all background tasks to complete.

The wait command returns the exit status of the last command of wait.

For example, to wait PID 7654 background processes, you will use:

Wait 7654Copy

When multiple processes are given, the command waits for all processes to complete.

Use jobspec to specify a job, which is a way to reference the processes that make up the job. Jobspec begins with a percentage symbol followed by the job number (% n). This is an example of ded shipped in the background:

Rsync-a / home / tmp/home & Copy

The shell job ID (enclosed in parentheses) and the process ID are displayed on the terminal:

[2] 54377Copy

Wait for the job, run the wait command followed by Jobspec:

Wait 2Copy

When called with the-n option, the command waits only for a single job to complete and return its exit status in the specified PID or Jobspecs. If no argument is provided, wait-n waits for any background job to complete and returns the job exit status.

Wait-n 45432 54346 76573Copy

In the above example, wait-n prints only the return status of the job that exits first; it does not show the PID of the process. If you want to get the process PID or Jobspec that returns the exit status, use the-p option to assign it to the variable:

Wait-p job_id-n 45432 54346 76573Copy

The-p option was introduced in BASH 5.1. If you use an older version of BASH, you will get a "invalid option" error.

The-f option tells wait to wait for each PID or JOBSPEC to terminate before returning its exit code, rather than when the job status is changed. This option is only valid when the job control is enabled. By default, job controls are enabled only for interactive prompts.

The example wait is typically used for shell scripts to generate child processes that are executed in parallel.

To illustrate how the command works, create the following script:

#! / bin/bash sleep 30 & processing idled cards! Echo "PID: $process_id" wait $process_id echo "Exit status: $?" Copy

Let's explain the code line by line:

The first line, called shebang, tells the operating system which interpreter to use to parse the rest of the file.

Use the sleep command to simulate a time-consuming background process.

$! Is an Bash internal variable that stores the PID of the last running process in the background. In this example, this is the PID of the sleep command. We store PID in a variable (process_id).

Print the pid number.

PID is passed to the wait command, which waits until the sleep command is complete.

Prints the exit status of the wait command. $? Is an internal Bash variable that holds the exit status of the last command executed.

If you run a script, it will print something like this:

PID: 36353 Exit status: 0Copy

Here is an example of using the-n option:

#! / bin/bash sleep 3 & sleep 30 & sleep 5 & wait-n echo "First job completed." Wait echo "All jobs completed." Copy

When the script is executed, it produces three background processes. Wait-n waits until the first job is completed and the echo statement is printed. Wait waits for all background tasks to complete.

First job completed all jobs completedCopy

The last example illustrates the-f option. Open the terminal and run:

Sleep 3600 & Copy

[1] 46671Copy

Wait for this process:

Wait 46671Copy

Open another terminal and use the kill command to stop the process:

Kill-STOP 46671Copy

Once the process state is changed, the wait command completes and returns the process exit code.

Now, repeat the same steps, but this time use wait-f $pid:

Sleep 3600 & wait-f 46671Copy

Stop the process of another terminal:

Kill-STOP 46671Copy

The wait command will not be completed this time. It will run before the sleep process terminates.

After reading the above, have you mastered the specific use of the Linux bash wait command? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.