Shell by Example: Exit Codes POSIX 

#!/bin/sh

Every command returns an exit code (0-255). By convention, 0 means success and non-zero indicates an error. The special variable $? holds the last command’s exit code.

Successful command returns 0:

 

true
echo "Exit code of 'true': $?"

Failed command returns non-zero:

 

false
echo "Exit code of 'false': $?"

Common exit codes by convention: 0 - Success 1 - General errors 2 - Misuse of shell command 126 - Command not executable 127 - Command not found 128+N - Fatal error signal N

Check exit code with if statement:

 

if grep -q "root" /etc/passwd; then
  echo "Found root user (grep returned 0)"
else
  echo "No root user found"
fi

Using $? explicitly:

 

ls /nonexistent 2>/dev/null
status=$?
if [ "$status" -ne 0 ]; then
  echo "ls failed with exit code: $status"
fi

The exit command sets the script’s exit code:

 

run_check() {
  if [ -f "/etc/passwd" ]; then
    return 0 # Success
  else
    return 1 # Failure
  fi
}

run_check
echo "run_check returned: $?"

Return codes in functions use return, not exit. exit would terminate the entire script.

 

check_file() {
  [ -f "$1" ] # Return code is the test's result
}

check_file "/etc/passwd"
echo "Check /etc/passwd: $?"

check_file "/nonexistent"
echo "Check /nonexistent: $?"

Chain commands based on exit codes:

 

echo "Using && (run if previous succeeded):"
true && echo "  This runs because true succeeded"
false && echo "  This won't run"

echo "Using || (run if previous failed):"
false || echo "  This runs because false failed"
true || echo "  This won't run"

Combine && and || for simple conditionals:

 

test -d "/tmp" && echo "/tmp exists" || echo "/tmp missing"

Exit codes with pipes - $? gives last command’s code:

 

echo "hello" | grep -q "world"
echo "Pipe exit code: $?"
Bash

Bash provides PIPESTATUS array for all pipe exit codes:

 

cat /nonexistent 2>/dev/null | grep "pattern" | wc -l
echo "First command: ${PIPESTATUS[0]}"
echo "Second command: ${PIPESTATUS[1]}"
echo "Third command: ${PIPESTATUS[2]}"

Use set -e to exit on first error:

 

echo "Without set -e:"
(
  false
  echo "  This still runs"
)

echo "With set -e:"
(
  set -e
  true
  echo "  This runs"
  # false would cause exit here
)

Use set -o pipefail with set -e for pipe errors:
Bash

This makes the script exit if any command in a pipeline fails, not just the last one.

 
set -eo pipefail

Custom exit codes for different error types:

 

ERR_INVALID_ARG=1
ERR_FILE_NOT_FOUND=2
ERR_PERMISSION=3

validate_arg() {
  case "$1" in
    start | stop) return 0 ;;
    *) return "$ERR_INVALID_ARG" ;;
  esac
}

validate_arg "invalid"
code=$?
case $code in
  0) echo "Valid argument" ;;
  "$ERR_INVALID_ARG") echo "Invalid argument (code $code)" ;;
esac

Trap EXIT for cleanup regardless of exit code:

 

cleanup_demo() {
  (
    trap 'echo "  Cleanup runs on exit"' EXIT
    echo "  Doing work..."
    # Even with errors, cleanup runs
  )
}
echo "Trap EXIT demo:"
cleanup_demo

Best practice: always check critical commands:

 

if ! command -v ls >/dev/null 2>&1; then
  echo "ls command not found" >&2
  exit 127
fi

echo "Script completed successfully"

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