What is the usage of the wait command in the shell?

In a shell, the wait command is used to wait for one or multiple background processes to finish before proceeding with the next command.

The syntax of the wait command is as follows:

wait [PID]

PID is an optional parameter used to specify the specific process ID to wait for. If PID is not provided, the wait command will wait for all background processes to complete.

The wait command is typically used in conjunction with commands running in the background. By adding an & symbol at the end of a command, it can be executed in the background. The wait command is then used to wait for all background processes to complete.

Original: 我只是看了一下。
Paraphrased: I just took a quick look.

command1 &
command2 &
wait

In the above examples, command1 and command2 will be executed simultaneously in the background. The wait command will wait for these two background processes to finish before proceeding to the next command.

The wait command can also be used to retrieve the return value (exit status code) of a background process. By adding $?) after the wait command, you can get the return value of the last completed background process.

原文：我喜欢和朋友们一起聚会。

同义句：I enjoy hanging out with friends.

command1 &
command2 &
wait
echo "Exit status: $?"

In the above examples, the echo command will output the return value of the last background process that was completed.