Shell Loops

Loops let you repeat things without copying and pasting. Need to rename a hundred files? Process every line in a log? Run the same command on ten servers? Loops are the answer. The shell has two main kinds: for and while.

The for loop

for loops over a list of items. The syntax is for var in list; do ...; done. The list can be words, filenames from a wildcard, or the output of a command.

#!/bin/bash
for fruit in apple banana cherry
do
    echo "I like $fruit"
done

Output:

I like apple
I like banana
I like cherry

Looping over files

The most common use of for is looping over files. Use a wildcard in the list and the shell expands it to matching filenames.

#!/bin/bash
for file in *.txt
do
    echo "Processing $file..."
    wc -l "$file"
done

The while loop

while loops keep going as long as a condition is true. They are great for reading files line by line, watching for changes, or repeating until a task finishes.

#!/bin/bash
count=1
while [ $count -le 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Break and continue

break exits the loop immediately. continue skips the rest of the current iteration and moves to the next one. These give you fine control over loop execution.

#!/bin/bash
for num in 1 2 3 4 5 6 7 8 9 10
do
    if [ $num -eq 3 ]
    then
        echo "Skipping 3"
        continue
    fi
    if [ $num -eq 8 ]
    then
        echo "Reached 8, stopping"
        break
    fi
    echo "Number: $num"
done

Output:

Number: 1
Number: 2
Skipping 3
Number: 4
Number: 5
Number: 6
Number: 7
Reached 8, stopping