Exit Codes — Communicating Success and Failure

# $? holds the exit code of the last command
ls /nonexistent
echo "Exit code: $?"    # 2 (ls error)

grep "pattern" file.txt
echo "Exit code: $?"    # 0=found, 1=not found, 2=error

# Use it in conditions
if grep -q "ERROR" logfile.txt; then
    echo "Errors found"
fi

# Check multiple commands
mkdir /tmp/work && cd /tmp/work && echo "In /tmp/work"
#  ^^^ only runs next command if previous succeeded

# Try something, fall back on failure
config_dir="${XDG_CONFIG_HOME:-$HOME/.config}"
mkdir -p "$config_dir" || { echo "Cannot create config dir" >&2; exit 1; }

#!/usr/bin/env bash

# exit N — terminates the script with code N
exit 0      # success
exit 1      # general error

# Without an explicit exit, the script exits with the code of the LAST command
echo "hello"
# implicit: exit 0 (echo succeeds)

grep "nonexistent" /etc/hosts
# implicit: exit 1 (grep found nothing)

# Best practice: always be explicit about exit codes
if ! some_command; then
    echo "some_command failed" >&2
    exit 1
fi
exit 0

#!/usr/bin/env bash

# Define exit codes as constants at the top
readonly E_OK=0
readonly E_USAGE=1
readonly E_MISSING_FILE=2
readonly E_PERMISSION=3
readonly E_NETWORK=4
readonly E_TIMEOUT=5

function backup_file {
    local src="$1"
    local dst="$2"

    [[ $# -ne 2 ]] && return $E_USAGE
    [[ ! -f "$src" ]] && return $E_MISSING_FILE
    [[ ! -r "$src" ]] && return $E_PERMISSION

    cp "$src" "$dst" || return $E_PERMISSION
    return $E_OK
}

backup_file "/etc/hosts" "/tmp/hosts.bak"
case $? in
    $E_OK)           echo "Backup done" ;;
    $E_MISSING_FILE) echo "Source file not found" >&2; exit $E_MISSING_FILE ;;
    $E_PERMISSION)   echo "Permission denied" >&2; exit $E_PERMISSION ;;
    *)               echo "Unknown error" >&2; exit 1 ;;
esac