A stack implementation, in bash

stack.sh

#!/bin/bash
# A stack, using bash arrays.
#
# Based on: https://gist.github.com/bmc/1323553 by Brian Clapper (bmc) <bmc@clapper.org>
# ---------------------------------------------------------------------------


# Check if a stack exists
#
# Usage: stack_exists name
stack_exists() {
    : ${1?'Missing stack name'}
    local _sname="${1}"

    eval '_i=$'"_stack_${_sname}_i"
    [[ -n "${_i}" ]]
}

# Create a new stack.
#
# Usage: stack_new name
#
# Example: stack_new x
stack_new() {
    : ${1?'Missing stack name'}
    local _sname="${1}"

    if stack_exists "${_sname}" ; then
        echo "Stack already exists -- '${_sname}'" >&2
        return 1
    fi

    # create main stack array
    eval "declare -ag _stack_${_sname}"
    # create stack offset variable
    eval "declare -ig _stack_${_sname}_i=0"

    return 0
}

# Destroy a stack
#
# Usage: stack_destroy name
stack_destroy() {
    : ${1?'Missing stack name'}
    local _sname="${1}"

    eval "unset _stack_${_sname}"{,_i}

    return 0
}

# Push one or more items onto a stack.
#
# Usage: stack_push stack item ...
stack_push() {
    : ${1?'Missing stack name'}
    : ${2?'Missing item(s) to push'}
    local _sname="${1}"
    shift 1
    local _items=( "${@}" )

    if ! stack_exists "${_sname}" ; then
        echo "No such stack -- '${_sname}'" >&2
        return 1
    fi

    eval "local _i=\${_stack_${_sname}_i}"

    local _item
    for _item in "${_items[@]}" ; do
        eval "_stack_${_sname}[${_i}]='${_item}'"
        let _i+=1
    done

    eval "let _stack_${_sname}_i=${_i}"

    return 0
}

# Print a stack to stdout.
#
# Usage: stack_print name
stack_print() {
    : ${1?'Missing stack name'}
    local _sname="${1}"

    if ! stack_exists "${_sname}" ; then
        echo "No such stack -- ${_sname}" >&2
        return 1
    fi

    echo "Stack '${_sname}':"

    local val
    eval "local _i=\${_stack_${_sname}_i}"
    while (( ${_i} > 0 )) ; do
        let _i-=1
        eval 'val=$'"{_stack_${_sname}[${_i}]}"
        printf "  [%02d] = '%s'\n" \
            "$((_i+1))" "${val}"
    done
}

# Get the size of a stack
#
# Usage: stack_size name var
#
# Example:
#    stack_size mystack n
#    echo "Size is $n"
stack_size() {
    : ${1?'Missing stack name'}
    : ${2?'Missing name of variable for stack size result'}
    local _sname="${1}"
    local _rname="${2}"

    if ! stack_exists "${_sname}" ; then
        echo "No such stack -- ${_sname}" >&2
        return 1
    fi

    eval "${_rname}=\${_stack_${_sname}_i}"
}

# Pop the top element from the stack.
#
# Usage: stack_pop name var
#
# Example:
#    stack_pop mystack top
#    echo "Got $top"
stack_pop() {
    : ${1?'Missing stack name'}
    : ${2?'Missing name of variable for popped result'}
    local _sname="${1}"
    local _rname="${2}"

    if ! stack_exists "${_sname}" ; then
        echo "No such stack -- ${_sname}" >&2
        return 1
    fi

    local _i
    eval 'let _i=$'"_stack_${_sname}_i"
    if [[ ${_i} == 0 ]] ; then
        echo "Empty stack -- ${_sname}" >&2
        return 1
    fi

    let _i-=1
    eval "${_rname}=\${_stack_${_sname}[${_i}]}"
    eval "unset _stack_${_sname}[${_i}]"
    eval "_stack_${_sname}_i=${_i}"

    return 0
}