Guide to the Linux Command 'Wait'

Control the flow of your scripts by telling processes to wait

Wait hand appearing out of Laptop computer screen

 Dimitri Otis/DigitalVision/Getty Images

The Linux 'wait' command lets you control the flow of your scripts and prevent conflicts by waiting for a process to complete or exit before moving on and executing the next line. It can be especially useful when you need something, whether it be a variable or file, from a previous command to work within the next part of the script.

Using the 'Wait' Command

The wait command is fairly simple. You give it a process ID number, and it will hold up the execution of your script until it receives an exit code from that process. In simpler terms. It will force your script to wait for the process you give it to finish. Then, your script will pick back up normally.

It's much easier to understand how wait works when you see it in action. Create a new script with the code below.

#! /bin/bash

function sleep_10 {
x=0
while [ $x -lt 10 ]; do
x=$[ $x+1 ]
sleep 1
echo 2nd sleep slept for $x seconds
done
echo "Waiting for first sleep..."
}

echo "Sleeping 20 seconds in the background..."
sleep 20 &
process_id1=$!
echo "Sleeping 10 seconds in the background..."
sleep_10 &
process_id2=$!
wait $process_id1
echo "Finished 20 second first sleep"
wait $process_id2
echo "Finished 10 second sleep"

The script is fairly simple. First, it defines a function that will count down ten seconds. Then, it starts by sleeping for twenty seconds as a background process. That process ID then gets saved to the variable "process_id1." Then, it executes the function, counting down through ten seconds and saves that process ID to "process_id2." When it's done, it will wait for the first twenty second sleep to complete in the background. When that's done, it'll wait for the second process to finish, but since that'll complete before the first one, it will announce that both sleep commands have completed.

Linux script using wait command

The results will look something like the image below.

Example of Linux wait command

There really isn't too much to the wait command, but it takes some planning to get it to execute the way you want.

'Wait' Command Manual

Below, you'll find the more technical manual page information for the wait command.

Name

wait, waitpid - wait for process termination

Synopsis

#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);

Description

The wait function suspends execution of the current process until a child has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.

The waitpid function suspends execution of the current process until a child as specified by the pid argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child as requested by pid has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.

The value of pid can be one of:

< -1

which means to wait for any child process whose process group ID is equal to the absolute value of pid.

-1

which means to wait for any child process; this is the same behavior which wait exhibits.

0

which means to wait for any child process whose process group ID is equal to that of the calling process.

> 0

which means to wait for the child whose process ID is equal to the value of pid.

The value of options is an OR of zero or more of the following constants:

WNOHANG

which means to return immediately if no child has exited.

WUNTRACED

which means to also return for children who are stopped, and whose status has not been reported.

(For Linux-only options, see below.)

If status is not NULLwait or waitpid store status information in the location pointed to bystatus.

This status can be evaluated with the following macros (these macros take the stat buffer (an int) as an argument — not a pointer to the buffer!):

WIFEXITED(status)

is non-zero if the child exited normally.

WEXITSTATUS(status)

evaluates to the least significant eight bits of the return code of the child which terminated, which may have been set as the argument to a call to exit() or as the argument for a return statement in the main program. This macro can only be evaluated if WIFEXITED returned non-zero.

WIFSIGNALED(status)

returns true if the child process exited because of a signal which was not caught.

WTERMSIG(status)

returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED returned non-zero.

WIFSTOPPED(status)

returns true if the child process which caused the return is currently stopped; this is only possible if the call was done using WUNTRACED.

WSTOPSIG(status)

returns the number of the signal which caused the child to stop. This macro can only be evaluated if WIFSTOPPED returned non-zero.

Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS) also define a macro WCOREDUMP(status) to test whether the child process dumped core. Only use this enclosed in #ifdef WCOREDUMP ... #endif.

Return Value

The process ID of the child which exited, or zero if WNOHANG was used and no child was available, or -1 on error (in which case errno is set to an appropriate value).

Errors

ECHILD

if the process specified in pid does not exist or is not a child of the calling process. (This can happen for one's own child if the action for SIGCHLD is set to SIG_IGN. See also the LINUX NOTES section about threads.)

EINVAL

if the options argument was invalid.

EINTR

if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.