Example bash script with option parsing, error handling, usage and cleanups

example.sh

#!/bin/bash

# Define your global variables
#
# Add names of all global variables to GLOBALS
GLOBALS="   GLOBALS \
            PATH \
            SCRIPT_NAME \
            ARG \
            OPTION \
            OPT_BOOL \
        "
PATH=/bin:/usr/bin              # Set lookup PATH for progs
SCRIPT_NAME=$(basename $0)      # Get script name
ARG=                            # declared global var with initial null value
OPTION="Some default value"     # same but with default value
OPT_BOOL=0                      # Boolean global var, with default value 0


# Function declarations

exit_script()
{
    # Method to clean environment and exit.
    # If first argument is a number between 0 and 255 it's treated as an
    # exit code to be used for exiting.
    # If the exit_code is not 0, all other arguments passed to exit_script()
    # are printed to stderr with "ERROR:" prefix. If the exit code is 0, then
    # the arguments are printed to stdout with "INFO: " prefix.

    # Default exit code is 1
    local exit_code=1
    local re var

    re='^([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])$'
    if echo "$1" | egrep -q "$re"; then
        exit_code=$1
        shift
    fi

    re='[[:alnum:]]'
    if echo "$@" | egrep -iq "$re"; then
        if [ $exit_code -eq 0 ]; then
            echo "INFO: $@"
        else
            echo "ERROR: $@" 1>&2
        fi
    fi

    # Clean-up global variables
    for var in $GLOBALS; do
        unset $var
    done

    # Print 'aborting' string if exit code is not 0
    [ $exit_code -ne 0 ] && echo "Aborting script..."

    exit $exit_code
}

usage()
{
    # Prints out usage and runs exit_script().
    # All args passed to usage() are forwarded to exit_script()
    
    sed -e "s/^    //" -e "s|SCRIPT_NAME|$SCRIPT_NAME|" <<"    EOF"
    USAGE

    Description of the script

    SYNTAX
            SCRIPT_NAME [OPTIONS] ARGUMENTS

    ARGUMENTS

     name                   Short description of the argument

    OPTIONS

     -o, --option <value>   Short description of the option and the value
     -b, --boolean          Short description of the option
     -h, --help             Prints this usage.

    EOF

    exit_script $@
}

test_arg()
{
    # Used to validate user input
    local arg="$1"
    local argv="$2"

    if [ -z "$argv" ]; then
        if echo "$arg" | egrep -q '^-'; then
            usage "Null argument supplied for option $arg"
        fi
    fi

    if echo "$argv" | egrep -q '^-'; then
        usage "Argument for option $arg cannot start with '-'"
    fi
}

# Your own functions go under here...



# MAIN() function to run the actual logic
main()
{
    # Set the default exit value to be 0
    local exit_value=0

    # Parse arguments and options
    [ $# -gt 0 ] || usage
    while [ $# -gt 0 ]; do
        case "$1" in
            -o|--option)
                test_arg "$1" "$2"
                shift
                OPTION="$1"
                shift
                ;;
            -b|--boolean)
                OPT_BOOL=1
                shift
                ;;
            -h|--help)
                usage 0
                ;;
            *)
                test_arg "$1"
                ARG="$1"
                shift
                ;;
        esac
    done

    # Your code goes here...


    # Exit script
    exit_script $exit_value
}

main $@
# EOF