Shell by Example: While Loops POSIX 

#!/bin/sh

The while loop executes commands as long as a condition is true. It’s essential for condition-based iteration.

Basic while loop counting from 1 to 5:

 

i=1
while [ "$i" -le 5 ]; do
  echo "Count: $i"
  i=$((i + 1))
done

The condition is tested before each iteration. If false initially, the loop body never runs.

Countdown example:

 

echo "Countdown:"
n=5
while [ "$n" -gt 0 ]; do
  echo "  $n"
  n=$((n - 1))
done
echo "  Liftoff!"

While loops are great for reading files line by line:

 

echo "Reading lines:"
while IFS= read -r line; do
  echo "  -> $line"
done <<'EOF'
First line
Second line
Third line
EOF

The IFS= prevents leading/trailing whitespace trimming. The -r flag prevents backslash interpretation.

Infinite loop with break:

 

count=0
while true; do
  count=$((count + 1))
  echo "Iteration $count"
  if [ "$count" -ge 3 ]; then
    echo "Breaking out"
    break
  fi
done

The continue statement skips to the next iteration:

 

echo "Skipping even numbers:"
i=0
while [ "$i" -lt 10 ]; do
  i=$((i + 1))
  # Skip even numbers
  if [ $((i % 2)) -eq 0 ]; then
    continue
  fi
  echo "  Odd: $i"
done

Reading from a command’s output:

 

echo "First 3 users:"
who | head -3 | while read -r user tty rest; do
  echo "  User: $user on $tty"
done

Note: Variables modified inside a piped while loop won’t persist outside due to subshell behavior.

Use process substitution or here-string to avoid subshells. In POSIX sh, redirect from a temp file or use a different approach.

While loop with multiple conditions:

 

x=0
y=10
while [ "$x" -lt 5 ] && [ "$y" -gt 5 ]; do
  echo "x=$x, y=$y"
  x=$((x + 1))
  y=$((y - 1))
done

The until loop is the opposite of while - it runs until the condition becomes true:

 

echo "Until loop:"
num=1
until [ "$num" -gt 3 ]; do
  echo "  num is $num"
  num=$((num + 1))
done

Menu-driven loop example:

 

echo "Simple menu loop:"
choice=""
attempt=0
while [ "$choice" != "q" ] && [ "$attempt" -lt 3 ]; do
  attempt=$((attempt + 1))
  choice="q" # Simulate user choosing quit
  echo "  Attempt $attempt: User chose '$choice'"
done

Processing arguments with while and shift:

 

set -- arg1 arg2 arg3
echo "Processing arguments:"
while [ $# -gt 0 ]; do
  echo "  Arg: $1"
  shift
done

While loop for retry logic:

 

max_retries=3
retry=0
success=false

while [ "$retry" -lt "$max_retries" ]; do
  retry=$((retry + 1))
  echo "Attempt $retry of $max_retries"

  # Simulate success on third try
  if [ "$retry" -eq 3 ]; then
    success=true
    break
  fi

  echo "  Failed, retrying..."
  sleep 1
done

if [ "$success" = true ]; then
  echo "Succeeded!"
else
  echo "All retries failed"
fi

« For Loops | Functions »

by Jose Diaz-Gonzalez | inspired by Go by Example | source