Skip to content

08-03 — Shell Scripting: Loops

Navigation: ← 08-02 — Shell Scripting: Conditionals | Notes Index | Next → 08-04 — Shell Scripting: Functions Exercise: Exercise 08-03

for Loop — Repeat for Each Item in a List

A for loop runs the same block of commands once for each item in a list.

for FRUIT in apple banana cherry; do
    echo "I like $FRUIT"
done

Output: 

I like apple
I like banana
I like cherry

Structure: - for VARIABLE in LIST; do — VARIABLE gets each value from LIST in turn - The commands between do and done run once per item - done marks the end of the loop

Loop over a range of numbers

for i in {1..5}; do
    echo "Number: $i"
done

Output: 

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

{1..5} is a brace expansion — it automatically generates the sequence 1 2 3 4 5.

You can also step by 2:

for i in {0..10..2}; do
    echo $i
done

Output: 0 2 4 6 8 10

Loop over files

for FILE in /var/log/*.log; do
    echo "Found log file: $FILE"
done

The * is a wildcard — matches any filename ending in .log. The loop runs once per matching file.

C-style for loop (like in C or Java)

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

This is equivalent to the {1..5} version but gives you more control over the step value.

while Loop — Repeat While a Condition is True

A while loop keeps running as long as the condition stays true.

COUNT=1
while [ $COUNT -le 5 ]; do
    echo "Count: $COUNT"
    ((COUNT++))    # add 1 to COUNT each iteration
done

Output: 

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

((COUNT++)) is shorthand for "add 1 to COUNT". The loop checks the condition at the top before each iteration — once COUNT becomes 6, [ 6 -le 5 ] is false and the loop stops.

Read a file line by line

A very common pattern — process each line of a file:

while read LINE; do
    echo "Line: $LINE"
done < /etc/passwd

< /etc/passwd feeds the file into the while read loop. Each iteration, read picks up the next line and stores it in LINE.

Wait for something to be ready

while ! nc -zv localhost 80 2>/dev/null; do
    echo "Waiting for web server..."
    sleep 2
done
echo "Web server is ready!"

! negates the condition — "while the web server is NOT ready, keep waiting". Once nc succeeds (port 80 is open), the ! makes the condition false and the loop exits.

until Loop — Opposite of while

until runs as long as the condition is false (stops when it becomes true):

COUNT=1
until [ $COUNT -gt 5 ]; do
    echo "Count: $COUNT"
    ((COUNT++))
done

This produces the same output as the while example above. until is less common — use whichever reads more naturally for your condition.

break and continue

  • break — exit the loop immediately
  • continue — skip the rest of this iteration and jump to the next
for i in {1..10}; do
    [ $i -eq 5 ] && continue    # skip 5
    [ $i -eq 8 ] && break       # stop the loop at 8
    echo $i
done

Output: 1 2 3 4 6 7

  • When i=5: continue is triggered, so echo is skipped, loop moves to i=6
  • When i=8: break is triggered, the entire loop ends

&& shorthand

[ condition ] && command is a shortcut for a one-line if:

[ $i -eq 5 ] && continue
# is the same as:
if [ $i -eq 5 ]; then
    continue
fi

Looping Over Command Output

for USER in $(cut -d: -f1 /etc/passwd); do
    echo "User: $USER"
done

$(cut -d: -f1 /etc/passwd) extracts the first field (usernames) from /etc/passwd and the loop iterates over each username.

Practical Example — Batch File Renamer

#!/bin/bash

# Rename all .txt files in current directory to .bak
COUNT=0

for FILE in *.txt; do
    [ -e "$FILE" ] || continue    # skip if no .txt files exist

    NEWNAME="${FILE%.txt}.bak"    # replace .txt extension with .bak
    mv "$FILE" "$NEWNAME"
    echo "Renamed: $FILE → $NEWNAME"
    ((COUNT++))
done

echo "Done. Renamed $COUNT file(s)."

${FILE%.txt} strips the .txt suffix from the filename — this is called parameter expansion.

Navigation: ← 08-02 — Shell Scripting: Conditionals | Notes Index | Next → 08-04 — Shell Scripting: Functions Exercise: Exercise 08-03