jaytaylor's Bash shell skeleton programming snippets quick reference

._bash_shell_programming_snippets.md

Bash shell skeleton programming snippets

Quick references to common and useful bash programming snippets and boilerplate.

See also: JayTaylor's Python boilerplate skeleton snippets

_bash_barebones_skeleton_standard_basic.sh

#!/usr/bin/env bash

main() {
    printf '%s\n' "INFO: $(basename "$0"):${FUNCNAME[0]}:${LINENO}: Hello world" 1>&2
}
export -f main

if [ "${BASH_SOURCE[0]}" = "${0}" ] || [ "${BASH_SOURCE[0]}" = '--' ]; then
    set -o errexit
    set -o pipefail
    set -o nounset

    if [ "${1:-}" = '-v' ]; then
        printf '%s\n' "INFO: $(basename "$0")::${LINENO}: Verbose output enabled" 1>&2
        shift
        set -o xtrace
    fi

    main "$@"
fi

bash_bare_bones_program_fancy_compact.sh

#!/usr/bin/env bash

_ts() { date -u +'%Y-%m-%dT%H:%M:%S%z'; }
log_debug() { printf '%s\n' "$(_ts) DEBUG: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2; }
log_info() { printf '%s\n' "$(_ts) INFO: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2; }
log_warn() { printf '%s\n' "$(_ts) WARN: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2; }
log_error() { printf '%s\n' "$(_ts) ERROR: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2; }
die() { log_error "$*"; exit 1; }

export -f _ts
export -f log_debug
export -f log_info
export -f log_warn
export -f log_error
export -f die

# n.b. alternate one-liner hack form:
# eval "$(for x in info warn error; do echo -e "log_${x}() { echo \"\$(_ts) $(echo ${x} | tr [:lower:] [:upper:]): \$*\"; };"; done)"

usage() { printf '%s\n' "usage: $0 [-v?] ..."; }

main() {
    if [[ "${1:-}" =~ ^(-h|--help)$ ]]; then
        usage
        return 0
    fi
}

if [ "${BASH_SOURCE[0]}" = "${0}" ] || [ "${BASH_SOURCE[0]}" = '--' ]; then
    set -o errexit
    set -o pipefail
    set -o nounset

    if [ "${1:-}" = '-v' ]; then
        log_debug 'verbose mode enabled'
        shift
        set -o xtrace
    fi

    main "$@"
fi

bash_bare_bones_program_fancy_readability.sh

#!/usr/bin/env bash

_ts() {
    date -u +'%Y-%m-%dT%H:%M:%S%z'
}
export -f _ts

log_debug() {
    printf '%s\n' "$(_ts) DEBUG: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2
}
export -f log_debug

log_info() {
    printf '%s\n' "$(_ts) INFO: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2
}
export -f log_info

log_warn() {
    printf '%s\n' "$(_ts) WARN: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2
}
export -f log_warn

log_error() {
    printf '%s\n' "$(_ts) ERROR: $(basename "$0"):$(caller 0 | awk '{ print $2 ":" $1 }') $*" 1>&2
}
export -f log_error

die() {
    log_error "$*"
    exit 1
}
export -f die

#export -f _ts
#export -f log_debug
#export -f log_info
#export -f log_warn
#export -f log_error
#export -f die

# n.b. alternate one-liner hack form:
# eval "$(for x in info warn error; do echo -e "log_${x}() { echo \"\$(_ts) $(echo ${x} | tr [:lower:] [:upper:]): \$*\"; };"; done)"

usage() {
    echo "usage: $0 [-v?] ..."
}

main() {
    if [[ "${1:-}" =~ ^(-h|--help)$ ]]; then
        usage
        return 0
    fi
}

# TODO: Revisit this, behavior seems different from what I observed on
#       Kubernetes last week.  Currently not working properly.
## n.b. Checking for '--' will activate main in cases where the script body has
##      been passed in as a string argument.
##
## Example:
##     bash -c 'echo "args=\"$@\" BASH_SOURCE[0]=\""${BASH_SOURCE}"\""' -- -v

if [ "${BASH_SOURCE[0]}" = "${0}" ] || [ "${BASH_SOURCE[0]}" = '--' ]; then
    set -o errexit
    set -o pipefail
    set -o nounset

    if [ "${1:-}" = '-v' ]; then
        printf '%s\n' 'INFO: Verbose output enabled' 1>&2
        shift
        set -o xtrace
    fi

    main "$@"
fi

bash_bare_bones_program_minimalist.sh

#!/usr/bin/env bash

die() {
    printf '%s\n' "ERROR: $*" 1>&2
    exit 1
}
export -f die

usage() {
    printf '%s\n' "usage: $0 [-v?] ..."
}

main() {
    if [[ "${1:-}" =~ ^(-h|--help)$ ]]; then
        usage
        return 0
    fi
}

if [ "${BASH_SOURCE[0]}" = "${0}" ] || [ "${BASH_SOURCE[0]}" = '--' ]; then
    set -o errexit
    set -o pipefail
    set -o nounset

    if [ "${1:-}" = '-v' ]; then
        printf '%s\n' 'INFO: Verbose output enabled' 1>&2
        shift
        set -o xtrace
    fi

    main "$@"
fi

bash_safe_find_for_loop.md

Safe for-loop for find output

https://stackoverflow.com/a/9612232/293064

find '.' -print0 | while IFS= read -d '' -r line; do
    echo "line=${line}"
done

bash_snippet_easy_verbose_mode_flag.md

Bash verbose xtrace mode

if [ "${1:-}" = '-v' ]; then
    printf '%s\n' 'DEBUG: verbose mode enabled' 1>&2
    set -o xtrace
    # n.b. trap is probably pointless here.  Use at your own discretion.
    trap 'set +o xtrace' EXIT
    shift
fi

bash_snippet_fancy_logging.md

Fancy log messaging with source file, function name, and line number incorporated.

# Logging message which includes the source file, function name, and line number.
printf '%s\n' "DEBUG: $(basename "${BASH_SOURCE}"):${FUNCNAME[0]}:${LINENO}: This is a log message" 1>&2

bash_snippet_get_script_directory.md

Obtain script directory

BASEPATH="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"

cd "${BASEPATH}"

bash_snippet_main.md

Bash main()

Bash equivalent to if __name__ == '__main__': in Python.

n.b. The '--' part needs to be revisited. Yesterday (Sunday 2020-02-09) I was unable to reproduce the behavior previously observed in Kubernetes a few weeks ago.

if [ "${BASH_SOURCE[0]}" = "${0}" ] || [ "${BASH_SOURCE[0]}" = '--' ]; then
    main "$@"
fi

bash_snippet_pop_last_arg.md

Bash pop last argument / reverse_shift

This can also be thought of as a reverse of the shift command.

Works for both scripts and functions.

Credit: Source

last="${@:$#}"

set -- "${@:1:$(($#-1))}"

bash_snippet_trap_append.md

trappend: Set or append command to trap signal handler

# trappend is like trap, with the addition  that if there is an existing trap
# already set, it will append the new command(s) without clobbering the
# pre-existing trap orders.
#
# n.b. Won't work for RETURN (hopefully this is somewhat obvious ;).
#
# usage: trappend cmds.. SIGNAL
trappend() {
    local sig
    local existing

    # n.b. Reverse-shift operation.
    sig="${*:$#}"
    set -- "${@:1:$(($#-1))}"

    if [ "${sig}" = 'RETURN' ]; then
        echo 'ERROR: trappend: SIGNAL value cannot be "RETURN"' 1>&2
        return 1
    fi

    if [ -n "$(trap -p "${sig}")" ]; then
        existing="$(trap -p "${sig}" | sed "s/^trap -- '\(.*\)' ${sig}\$/\1/");"
    fi

    # shellcheck disable=SC2064
    trap "${existing:-}$*" "${sig}"
}

Example usage

trappend 'echo hello 3' EXIT
trappend 'echo hello 4' EXIT

echo hello 0
echo hello 1

Output:

hello 0
hello 1
hello 3
hello 4