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Loops in Shell Scripting

Loops execute a block of code repeatedly until a certain condition is met. Bash supports three main types of loops: for, while, and until.

The for Loop​

The for loop iterates over a list of items (like words in a string, files in a directory, or numbers in a sequence).

Iterating over a list​

for COLOR in red green blue; do
    echo "The color is $COLOR"
done

Iterating over files​

for FILE in *.txt; do
    echo "Found text file: $FILE"
done

C-style for loop​

for (( i=1; i<=5; i++ )); do
    echo "Iteration number $i"
done

The while Loop​

The while loop executes as long as the condition remains true.

COUNTER=1

while [ "$COUNTER" -le 5 ]; do
    echo "Counter is $COUNTER"
    ((COUNTER++))
done

Reading a file line by line​

A very common use case for while loops is processing text files.

while read -r LINE; do
    echo "Processing line: $LINE"
done < input.txt

The until Loop​

The until loop is the opposite of the while loop. It executes until the condition becomes true (i.e., it runs while the condition is false).

COUNT=1

until [ "$COUNT" -gt 5 ]; do
    echo "Count is $COUNT"
    ((COUNT++))
done

Loop Control: break and continue​

  • break: Exits the loop entirely.
  • continue: Skips the rest of the current iteration and moves to the next one.
for i in {1..10}; do
    if [ "$i" -eq 5 ]; then
        continue # Skip 5
    fi
    if [ "$i" -eq 8 ]; then
        break # Stop at 8
    fi
    echo "Number $i"
done

Best Practices​

  • Always update the loop control variable in while or until loops to prevent infinite loops.
  • Use while read -r to read files, as it prevents backslash escaping issues.
  • Be careful with for var in $(command) as it breaks on spaces; prefer a while read loop for parsing lines.