Bash XML parser and stack, plus Gmail XML feed parsing example

bash-stack.bash

#!/usr/bin/env bash

# Bash Stack

set -o errtrace
set -o errexit
set -o nounset

declare -a stack

stack_push () {
    [ $# -eq 1 ] || _stack_err "stack_push takes one argument" || return 1
    stack[${#stack[@]}]="$1"
}
stack_pop () {
    index=$(_stack_index) || _stack_err "index out of range" || return 1
    unset stack[$index]
}
stack_get () {
    [ $# -ge 1 ] && item="$1" || item="0"
    index=$(_stack_index $item) || _stack_err "index out of range" || return 1
    echo ${stack[$index]}
}
stack_list () {
    echo ${stack[@]}
}
stack_size () {
    echo ${#stack[@]}
}

# internal:
_stack_index () {
    [ $# -ge 1 ] && index="$1" || index="0" || true
    [ "$index" -ge 0 ] && [ "$index" -lt ${#stack[@]} ] || return 1
    expr ${#stack[@]} - "$index" - 1 || true
}
_stack_err () {
    echo "Stack error: $@" 1>&2
    return 1
}

bash-xml.bash

#!/usr/bin/env bash

# Bash XML parser (SAX)
#
#   EXAMPLE: echo '<foo><bar>hello world</bar></foo>' | ./bash-xml.bash 'echo START:' 'echo END:' 'echo TEXT:'
#

set -o errtrace
set -o errexit
set -o nounset

bash_xml_sax_parse () {
    handle_element_start="$1"
    handle_element_end="$2"
    handle_characters="$3"
    # assumes each line contains one element
    cat "/dev/stdin" | bash_xml_split | while read line; do
        case "$line" in
            "<?"*)      ;;
            "</"*)      [ -z "$handle_element_end" ]    || $handle_element_end      "$line" "$(expr "$line" : '</*\([^ />]*\)')" ;;
            "<"*"/>")   [ -z "$handle_element_start" ]  || $handle_element_start    "$line" "$(expr "$line" : '</*\([^ />]*\)')"
                        [ -z "$handle_element_end" ]    || $handle_element_end      "$line" "$(expr "$line" : '</*\([^ />]*\)')" ;;
            "<"*)       [ -z "$handle_element_start" ]  || $handle_element_start    "$line" "$(expr "$line" : '</*\([^ />]*\)')" ;;
            *)          [ -z "$handle_characters" ]     || $handle_characters       "$line" ;;
        esac
    done
}

# splits an XML document into a stream of lines containing one element each and removes blanks
# TODO: make this more robust
bash_xml_split () {
    sed -e 's/</\
</g' -e 's/>/>\
/g' | sed -e '/^ *$/d'
}

if [ "$0" == "$BASH_SOURCE" ]; then
    bash_xml_split | bash_xml_sax_parse "$@"
fi

gmail.bash

#!/usr/bin/env bash

# Gmail inbox feed parser
#
#   EXAMPLE: ./gmail.bash GMAIL_USER
#
# GMAIL_USER can include username and optionally a password

set -o errtrace
set -o errexit
set -o nounset

parent=$(dirname -- "$BASH_SOURCE")
source "$parent/bash-xml.bash"
source "$parent/bash-stack.bash"

gmail_parse_feed () {
    bash_xml_sax_parse gmail_parse_feed_element_start gmail_parse_feed_element_end gmail_parse_feed_characters
}
gmail_parse_feed_element_start () {
    [ "$2" != "entry" ] || (gmail_entry_name="" && gmail_entry_title="" && gmail_entry_date="")
    stack_push "$2"
}
gmail_parse_feed_element_end () {
    [ "$2" != "entry" ] || printf "%20s | %-25s | %s\n" "$gmail_entry_date" "$gmail_entry_name" "$gmail_entry_title"
    stack_pop
}
gmail_parse_feed_characters () {
    case "$(stack_list)" in
        "feed entry title")       gmail_entry_title="$1" ;;
        "feed entry issued")      gmail_entry_date="$1"  ;;
        "feed entry author name") gmail_entry_name="$1"  ;;
    esac
}

gmail_fetch_feed () {
    curl -u "$1" --silent 'https://mail.google.com/mail/feed/atom'
}

gmail_inbox () {
    if [ $# -gt 0 ]; then
        GMAIL_USER="$1"
    fi
    gmail_fetch_feed "$GMAIL_USER" | gmail_parse_feed
}

if [ "$0" == "$BASH_SOURCE" ]; then
    gmail_inbox "$@"
fi

xml-prettify.bash

#!/usr/bin/env bash

# XML prettifier
#
#   EXAMPLE: echo '<foo><bar>hello world</bar></foo>' | ./xml-prettify.bash
#

set -o errtrace
set -o errexit
set -o nounset

parent=$(dirname -- "$BASH_SOURCE")
source "$parent/bash-xml.bash"
source "$parent/bash-stack.bash"

xml_prettify_indent="2"
xml_prettify_print () {
    printf "%*s%s\n" $(expr $(stack_size) \* $xml_prettify_indent) "" "$1"
}

xml_prettify () {
    bash_xml_sax_parse xml_prettify_start xml_prettify_end xml_prettify_characters
}

xml_prettify_start () {
    case "$1" in
        *"/>") ;; # HACK: skip self terminating tags
        *) xml_prettify_print "$1" ;;
    esac
    stack_push "$2"
}
xml_prettify_end () {
    stack_pop
    xml_prettify_print "$1"
}
xml_prettify_characters () {
    xml_prettify_print "$1"
    return 0
}

if [ "$0" == "$BASH_SOURCE" ]; then
    xml_prettify
fi