Bash Until Loop
This tutorial explains the basics of the until loop in Bash.
Loops are one of the fundamental concepts of programming languages. Loops are handy when you want to run a series of commands over and over again until a specific condition is met.
In scripting languages such as Bash, loops are useful for automating repetitive tasks. There are 3 basic loop constructs in Bash scripting, for loop , while loop , and until loop.
Bash until Loop
The until loop is used to execute a given set of commands as long as the given condition evaluates to false.
The Bash until loop takes the following form:
until [CONDITION] do [COMMANDS] done
The condition is evaluated before executing the commands. If the condition evaluates to false, commands are executed. Otherwise, if the condition evaluates to true the loop will be terminated and the program control will be passed to the command that follows.
In the example below, on each iteration the loop prints the current value of the variable counter and increments the variable by one.
#!/bin/bash counter=0 until [ $counter -gt 5 ] do echo Counter: $counter ((counter++)) done
The loop iterates as long as the counter variable has a value greater than four. The script will produce the following output:
Counter: 0 Counter: 1 Counter: 2 Counter: 3 Counter: 4 Counter: 5
Use the break and continue statements to control the loop execution.
Bash until Loop Example
The following script may be useful when your git host has downtime, and instead of manually typing git pull multiple times until the host is online, you can run the script once. It will try to pull the repository until it is successful.
#!/bin/bash
until git pull &> /dev/null
do
echo "Waiting for the git host ..."
sleep 1
done
echo -e "\nThe git repository is pulled."
The script will print “Waiting for the git host …” and sleep for one second until the git host goes online. Once the repository is pulled, it will print “The git repository is pulled.”.
Waiting for the git host ... Waiting for the git host ... Waiting for the git host ... The git repository is pulled.
Conclusion
The while and until loops are similar to each other. The main difference is that the while loop iterates as long as the condition evaluates to true and the until loop iterates as long as the condition evaluates to false.
