1. Background
When a user issues multiple command sequences in Linux, the commands are executed immediately one after another or simultaneously (for example, the tee command). However, sometimes it is necessary to postpone the execution of the command and provide the system with sufficient time to produce the expected results.
2. Introduction
The sleep command suspends the calling process of the next command for a specified period of time. This property is useful when the execution of the following command depends on the successful completion of the previous command.
3. Syntax
sleep [number]
By default, the number of sleep after the system reads is used as the number of seconds. To specify additional time units, use the following syntax:
sleep [number][unit]
sleep 1h 2m 0.5s
The sleep command accepts floating point numbers. It allows multiple values to be added to calculate sleep.
Available units are:
- s-seconds
- m-minute
- h-hour
- d-day
To stop sleep after the start and before the specified waiting time ends, press Ctrl + C.
To view the help of the sleep command, type:
sleep --help
For version details, type:
sleep --version
4. Difference from wait
bash wait commandIs a shell command that waits for the process running in the background to complete and returns to the exit status. Unlike the sleep command that waits for a specified time, the wait command waits for all or specific background tasks to complete.
5. Example
5.1 Setting up an alarm
Used to sleep tell the system to play mp3 files after a certain time.
sleep 7h 30m && mplayer alarm.mp3
5.2 Delay commands in the terminal
Sleep is useful for enforcing the time between two commands, executing at one second interval:
$ sleep 1 && echo "one" && sleep 1 && echo "two" one two
5.3 Variable assignment to sleep
Variables can be assigned to the sleep command.
#!/bin/bash
SLEEP_INTERVAL="30"
CURRENT_TIME=$(date +"%T")
echo "Time before sleep: ${CURRENT_TIME}"
echo "Sleeping for ${SLEEP_INTERVAL} seconds"
sleep ${SLEEP_INTERVAL}
CURRENT_TIME=$(date +"%T")
echo "Time after sleep: ${CURRENT_TIME}"
The script defines a variable named SLEEP_INTERVAL, whose value is later used as an argument to the sleep command. The output of this sample script shows that execution lasted for 30 seconds:
$ ./time_script.sh Time before sleep: 00:01:15 Sleeping for 30 seconds Time after sleep: 00:01:45
5.4 Define the check interval
Check if the website is online, if a website is successfully pinged, the script will stop, introducing a 10-second delay between unsuccessful pings.
#!/bin/bash
while :
do
if ping -c 1 &> /dev/null
then
echo "Google is online"
break
fi
sleep 10
done
5.5 Set aside time for operation completion
You may be running a bash script that calls two other bash scripts internally—one running the test in the background, and the other printing the result. If the second script is executed before the first script is completed, it is used to sleep to prevent the second script from printing errors:
while kill -0 $BACK_PID ; do
echo "Waiting for the process to end"
sleep 1
done
The kill -0 $BACK_PID command checks whether the process of the first script is still running. If yes, it prints the message and hibernates for 1 second, then checks again.
5.6 Predicted delay
Used to sleep to allow the execution of certain commands.
for (( i = 1 ; i <= 250 ; i++ ));
do
sleep 1
<do something>
done
refer to:
How to Use the Linux sleep Command with Examples
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