Bash can provides different programming language structures like variable, array, loop, decision making etc. Loop is one of the most useful feature of bash scripting. Loops will give the ability to iterate over given sequential structures like array, file list, numbers, inputs etc.
Loop Syntax
Bash provides alternative syntax for loop. Below are some examples of for loop syntax.
1 2 3 4 5 6 | for VARIABLE in 1 2 3 4 5 .. N do command1 command2 commandN done |
OR
1 2 3 4 5 6 | for VARIABLE in file1 file2 file3 do command1 on $VARIABLE command2 commandN done |
OR
1 2 3 4 5 6 | for OUTPUT in $(Linux-Or-Unix-Command-Here) do command1 on $OUTPUT command2 on $OUTPUT commandN done |
For line provides the loop logic and take step for each number or file or command output
Lines between do and done is the part which will run each step.
Loop Items
One of the most popular usage of loop is looping through items. We will provides items by separating them with space. All items will be looped. In this example we will use numbers from 1 to 7 and define them one by one.
1 | $ for count in 1 2 3 4 5 6 7 ; do echo $count; done |
We can loop numbers like above but the problem is how can we achieve if need to loop over 1000 numbers. Should we write the one by one? No we can specify number ranges like below..
1 | $ for count in {1..7} ; do echo $count; done |
We use {1..7} to loop from 1 to 7 one by one.
Increment Value
We can also specify increment number. Incrementing one by one is default behavior. How can we increment 2?
1 | $ for count in {1..5..2} ; do echo $count; done |
We have incremented loop 2 and only used numbers 1,3,5 . There is an alternative way to increment for loop with 43 for statement
C Style For Loop
We can specify a for statement like C programming language. We will set stat numbers, end condition and increment value like below. Here is the syntax
1 2 3 4 5 | for (( EXP1; EXP2; EXP3 )) do command1 ... done |
Now we will start from 1 and increment one by one to the 5.
1 | $ for (( count=1;count<6;count++)) ; do echo $count; done |
Infinite For Loop
We can create a loop for infinity. This loop will run forever if we do not end process.
1 | $ for (( ; ; )) ; do echo "Kill me if you can ;)"; done |
We do not provide any start number , end condition or increment value to trigger infinite loop.
Infinite While Loop
We can create infinite loop with while statement. Creating infinite loop while statement is more easy.
1 | $ while true ; do echo "Kill me if you can ;)"; done |
Just provide true statement to the while.
Loop Through Files
One of the most used type of loop is looping through files in specified directory. All file names in the specified directory will be looped step by step.
1 | $ for file in $(ls -1) ; do echo $file; done |
Here the $(ls -l) provides list of filenames to loop and each file names is assigned to the file variable for each step.
