Bash loop examples | Linux
When a command or series of commands needs to be repeated, it is put inside a loop. The shell provides three types of loop: while, until, and for. The first two execute until a condition is either true or false; the third loops through a list of words.
while
The condition for a while loop is a list of one or more commands, and the commands to be executed while the condition remains true are placed between the keywords do and done:
while <list> do <list> done
By incrementing a variable each time the loop is executed, the commands can be run a specific number of times:
n=1 while [ $n -le 10 ] do echo "$n" n=$(( $n + 1 )) done
The true command can be used to create an infinite loop:
while true ## : can be used in place of true do read x done
A while loop can be used to read line by line from a file:
while IFS= read -r line do : do something with "$line" done < FILENAMEy?
until
Rarely used, until loops as long as the condition fails. It is the opposite of while:
n=1 until [ $n -gt 10 ] do echo "$n" n=$(( $n + 1 )) done
for
At the top of a for loop, a variable is given a value from a list of words. On each iteration, the next word in the list is assigned:
for var in Canada USA Mexico do printf "%s\n" "$var" done
bash also has a nonstandard form that is similar to that found in the C programming language. The first expression is evaluated when first encountered. The second is a test. The third is evaluated after each iteration:
for (( n=1; n<=10; ++n )) do echo "$n" done
Since this offers no advantage over standard looping methods, it is not used in this book.
break
A loop can be exited at any point with the break command:
while : do read x [ -z "$x" ] && break done
With a numeric argument, break can exit multiple nested loops:
for n in a b c d e do while true do if [ $RANDOM -gt 20000 ] then printf . break 2 ## break out of both while and for loops elif [ $RANDOM -lt 10000 ] then printf '"' break ## break out of the while loop fi done done echo
continue
Inside a loop, the continue command immediately starts a new iteration of the loop, bypassing any remaining commands:
for n in {1..9}
do
x=$RANDOM
[ $x -le 20000 ] && continue echo "n=$n x=$x"
done
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