retry – retry a command until it succeeds

retry

#!/bin/bash

# Unofficial Bash Strict Mode (http://redsymbol.net/articles/unofficial-bash-strict-mode/)
set -euo pipefail
IFS=$'\n\t'
PROG="$(basename "$0")"
VERSION="0.1.0"
UPSTREAM_URL='https://gist.github.com/a-ludi/326a58b5a6b85c9c523833f614090438'


function set_defaults()
{
    ERROR_EXIT="${ERROR_EXIT:-42}"
    NUM_RETRIES=3
}


function error()
{
    if (( $# > 0 ));
    then
        echo "$PROG: error: $@"
    else
        echo
    fi
} >&2


function bail_out()
{
    [[ -v IS_SILENT ]] && exit "$ERROR_EXIT"

    error "$@"
    exit "$ERROR_EXIT"
}


function bail_out_usage()
{
    [[ -v IS_SILENT ]] && exit "$ERROR_EXIT"

    error "$@"
    error
    set_defaults
    print_usage
    exit "$ERROR_EXIT"
}


function warn()
{
    [[ -v IS_SILENT ]] || echo "$PROG: [WARNING] $@"
} >&2


function print_usage()
{
    echo "USAGE:  $PROG [-eh] [-n<uint($NUM_RETRIES)>] [--] <command>..."
} >&2


function print_help()
{
    print_usage
    echo
    echo 'This program executes <command> over and over again until it succeeds or the'
    echo 'maximum number of retries is reached.'
    echo
    echo 'Positional arguments:'
    echo ' <command>...       A shell command to be executed.'
    echo
    echo 'Optional arguments:'
    echo " --eval, -e         Use 'eval' to execute <command>."
    echo " --num-retries, -n  Maximum number of retries (default: $NUM_RETRIES)."
    echo ' --help, -h         Prints this help.'
    echo ' --usage            Print a short command summary.'
    echo ' --silent, -s       Supress any output; even errors and warnings.'
    echo ' --version          Print software version.'
    echo
    echo 'Environment:'
    echo " ERROR_EXIT         Exit code if an error in '$PROG' happens."
    echo ' LAST_EXIT          Last exit code of <command>; initially set to 0.'
    echo ' RETRY_ITERATION    Number of failed iterations. This is exported to <command>'
    echo '                    and may be used to alter its parameters, e.g. allocate'
    echo '                    more resuorces.'
} >&2


function print_version()
{
    echo "$PROG v$VERSION"
    echo
    echo 'This software is available for downloads at'
    echo
    echo "  $UPSTREAM_URL"
    echo
    echo
    echo "Copyright (c) 2020, Arne Ludwig <arne.ludwig@posteo.de>"
    echo
    echo 'Permission is hereby granted, free of charge, to any person obtaining a copy'
    echo 'of this software and associated documentation files (the "Software"), to deal'
    echo 'in the Software without restriction, including without limitation the rights'
    echo 'to use, copy, modify, merge, publish, distribute, sublicense, and/or sell'
    echo 'copies of the Software, and to permit persons to whom the Software is'
    echo 'furnished to do so, subject to the following conditions:'
    echo
    echo 'The above copyright notice and this permission notice shall be included in all'
    echo 'copies or substantial portions of the Software.'
    echo
    echo 'THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR'
    echo 'IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,'
    echo 'FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE'
    echo 'AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER'
    echo 'LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,'
    echo 'OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE'
    echo 'SOFTWARE.'
} >&2


function parse_args()
{
    for ARG in "$@";
    do
        case "$ARG" in
            -e|--eval)
                USE_EVAL=1
                shift
                ;;
            -n*|--num-retries=*)
                NUM_RETRIES="${ARG#-n}"
                NUM_RETRIES="${NUM_RETRIES#--num-retries=}"
                shift
                ;;
            -h|--help)
                print_help
                exit
                ;;
            --usage)
                print_usage
                exit
                ;;
            --silent|-s)
                IS_SILENT=1
                shift
                ;;
            --version)
                print_version
                exit
                ;;
            --)
                shift
                break
                ;;
            *)
                break
                ;;
        esac
    done

    (( $# > 0 )) || bail_out_usage 'error: missing <command>' >&2
    (( "$NUM_RETRIES" >= 0 )) || bail_out_usage 'error: --num-retries must be a non-negative integer' >&2
    COMMAND=( "$@" )

    (( "$NUM_RETRIES" > 0 )) || warn '<command> is not being executed because --num-retries=0' >&2
}


function run_command()
{
    if [[ -v USE_EVAL ]];
    then
        eval "${COMMAND[@]}"
    else
        "${COMMAND[@]}"
    fi
}


function main()
{
    set_defaults
    parse_args "$@"

    export RETRY_ITERATION=0
    export LAST_EXIT=0
    while (( RETRY_ITERATION < NUM_RETRIES )); do
        run_command && exit
        LAST_EXIT="$?"
        (( ++RETRY_ITERATION ))
    done

    exit "$LAST_EXIT"
}


main "$@"