Metaprogramming with Bash

Most programmers do not take full advantage of the languages they work in, though some languages make this a real challenge. Take metaprogramming, or programs that have some self-knowledge. LISP-family languages make this easy and natural; those with macros even more so. Bytecode languages (think Java), and even more so object code languages (think "C"), fall back on extra-linguistic magic such as AOP rewriting.

Text-based languages lay in a middle ground. Best known is Bash. Rarely do programmers take full advantage of Bash features, and few would think of metaprogramming. Not as clean as LISP macros, it is still straight forward.

As an example of function rewriting note line 21. The surrounding function body redefines an existing function, incorporating the original's body into itself. This is very similar to aspect-oriented programming with a "around" advice. Much care is taken to preserve the original function context.

This is a fully working script—give it a try!

#!/bin/bash

export PS4='+${BASH_SOURCE}:${LINENO}: ${FUNCNAME[0]:+${FUNCNAME[0]}(): } '

pgreen=$(printf "\e[32m")
pred=$(printf "\e[31m")
pboldred=$(printf "\e[31;1m")
preset=$(printf "\e[0m")
pcheckmark=$(printf "\xE2\x9C\x93")
pballotx=$(printf "\xE2\x9C\x97")
pinterrobang=$(printf "\xE2\x80\xBD")

function _register {
    case $# in
    1 ) local -r name=$1 arity=0 ;;
    2 ) local -r name=$1 arity=$2 ;;
    esac
    read -d '' -r wrapper <<EOF
function $name {
    # Original function
    $(declare -f $name | sed '1,2d;$d')

    __e=\$?
    shift $arity
    if (( 0 < __e || 0 == \$# ))
    then
        __tally \$__e
    else
        "\$@"
        __e=\$?
        __tally \$__e
    fi
}
EOF
    eval "$wrapper"
}

let __passed=0 __failed=0 __errored=0
function __tally {
    local -r __e=$1
    $__tallied && return $__e
    __tallied=true
    case $__e in
    0 ) let ++__passed ;;
    1 ) let ++__failed ;;
    * ) let ++__errored ;;
    esac
    _print_result $__e
    return $__e
}

function _print_result {
    local -r __e=$1
    case $__e in
    0 ) echo -e $pgreen$pcheckmark$preset $_scenario_name ;;
    1 ) echo -e $pred$pballotx$preset $_scenario_name ;;
    * ) echo -e $pboldred$pinterrobang$preset $_scenario_name ;;
    esac
}

function check_exit {
    (( $? == $1 ))
}

function make_exit {
    local -r e=$1
    shift
    (exit $e)
    "$@"
}

function check_d {
    [[ $PWD == $1 ]]
}

function change_d {
    cd $1
}

function variadic {
    :
}

function early_return {
    return $1
}

function eq {
    [[ "$bob" == nancy ]]
}

function normal_return {
    (exit $1)
}

function f {
    local -r bob=nancy
}

function AND {
    "$@"
}

function SCENARIO {
    local -r _scenario_name="$1"
    shift
    local __tallied=false
    local __e=0
    pushd $PWD >/dev/null
    "$@"
    __tally $?
    popd >/dev/null
}

_register f
_register normal_return 1
_register variadic 1
_register eq 
_register early_return 1
_register change_d 1
_register check_exit 1

echo "Expect 10 passes, 4 failures and 4 errors:"
SCENARIO "Normal return pass direct" normal_return 0
SCENARIO "Normal return fail direct" normal_return 1
SCENARIO "Normal return error direct" normal_return 2
SCENARIO "Normal return pass indirect" f AND normal_return 0
SCENARIO "Normal return fail indirect" f AND normal_return 1
SCENARIO "Normal return error indirect" f AND normal_return 2
SCENARIO "Early return pass indirect" f AND early_return 0
SCENARIO "Early return fail indirect" f AND early_return 1
SCENARIO "Early return error indirect" f AND early_return 2
SCENARIO "Early return pass direct" early_return 0
SCENARIO "Early return fail direct" early_return 1
SCENARIO "Early return error direct" early_return 2
SCENARIO "Variadic with none" f AND variadic
SCENARIO "Variadic with one" f AND variadic apple
SCENARIO "Local vars" f AND eq
here=$PWD
SCENARIO "Change directory" change_d /tmp
SCENARIO "Check directory" check_d $here
SCENARIO "Check exit" make_exit 1 AND check_exit 1

(( 0 == __passed )) || ppassed=$pgreen
(( 0 == __failed )) || pfailed=$pred
(( 0 == __errored )) || perrored=$pboldred
cat <<EOS
$ppassed$__passed PASSED$preset, $pfailed$__failed FAILED$preset, $perrored$__errored ERRORED$preset
EOS

Practical use of this script as a test framework would pull out SCENARIO and AND to a separate source script, included with "." or source, put the registered functions and their helpers in another source script, and provide command-line parsing to pick out which tests to execute. test-bitsh.sh is an example in progress.

Color your world

My coworkers use many ad hoc or single-purpose scripts, things like: checking system status, wrappers for build systems, launching services locally, etc. My UNIX background tells me, "keep it simple, avoid output; Silence is Golden."

Somehow my younger colleagues aren't impressed.

So to avoid acting my age I started sprinkling color into my scripts, and it worked. Feedback was uniformly positive. And true to my UNIX roots, I provided command line flags to disable color.

Some lessons for budding BASHers:

1. Yes, experiment and learn, but be sure to do your research. The Internet has outstanding help for BASH.
2. Learn standard idioms (see below).
3. Don't overdo it. Color for summary lines and warnings have more impact when the rest of the text is plain.
4. Keep functions small, just as you would in other languages. BASH is a programming language with a command line, so keep your good habits when writing shell.
5. Collaborate, pair! This comes naturally to my fellows. Coding is more enjoyable, goes faster and has fewer bugs.

Idioms

Most of these idioms appear in testing with bash, a simple BASH BDD test framework I wrote for demonstration.

Process command-line flags

this=false
that=true
while getopts :htT-: opt
do
    [[ - == $opt ]] && opt="${OPTARG%%=*}" OPTARG="${OPTARG#*=}"
    case $opt in
    h | help ) print_help ; exit 0 ;;
    t | this ) this=true ;;
    T | no-that ) that=false ;;
    esac
done
shift $((OPTIND - 1))

Keep boolean toggles simple

run_faster=true

if $run_faster
then
    use_faster_algorithm "$@"
else
    use_more_correct_algorithm "$@"
fi

Simple coloring

pgreen="\e[32m"
pred="\e[31m"
preset="\e0m"

if $success
then
    echo -e "${pgreen}PASS${preset} $test_name"
else
    echo -e "${pred}FAIL${preset} $test_name - $failure_reason"
EOM
fi

Consistent exit codes

function check_it {
    local -r failed=$1
    local -r syntax_error=$2
    if $syntax_error
    then
        return 2
    elif $failed
    then
        return 1
    else
        return 0
    fi
}

Coda

There are many more idioms to learn, hopefully this taste catches your interest. I was careful to include others mixed in with these (what does local -r do?) to whet the appetite for research. Go try the BASH debugger sometime.

UPDATE: Fixed thinko. ANSI escape codes need to be handled by echo -e or printf, not sent directly via cat!

BDD-style fluent testing in BASH

I wanted to impress on my colleagues that BASH was still hip, still relevant. And that I wasn't an old hacker. So I wrote a small BDD test framework for BASH.

Techniques

Fluent coding relies on several BASH features:

• Variable expansion happens before executing commands
• A shell function is indistinguishable from a program: they are called the same way
• Local function variables are dynamically scoped but only within a function, so are visible to other functions called within that scope, directly or indirectly through further function calls

Together with Builder pattern, it's easy to write given/when/then tests. (Builder pattern here solves the problem not of telescoping constructors, but massive, arbitrary argument lists.)

So when you run:

function c {
    echo "$message"
}

function b {
    "$@"
}

function a {
    local message="$1"
    shift
    "$@"
}

a "Bob's your uncle" b c

You see the output:

Bob's your uncle

How does this work?

First BASH expands variables. In function a this means that after the first argument is remembered and removed from the argument list, "$@" expands to b c. Then b c is executed.

Then BASH calls the function b with argument "c". Similarly b expands "$@" to c and calls it.

Finally as $message is visible in functions called by a, c prints the first argument passed to a (as it was remebered in the variable $message), or "Bob's your uncle" in this example.

Running the snippet with xtrace makes this clear (assuming the snippet is saved in a file named example):

bash -x example
+ a 'Bob'\''s your uncle' b c
+ local 'message=Bob'\''s your uncle'
+ shift
+ b c
+ c
+ echo 'Bob'\''s your uncle'
Bob's your uncle

So the test functions for given_jar, when_run and then_expect (along with other, similar functions) work the same way. Keep this in mind.

Application

So how does this buy me fluent BDD?

Given these functions:

function then_expect {
    local expectation="$1"
    shift

    if [[ some_test "$pre_condition" "$condition" "$expectation" ]]
    then
        echo "PASS: $scenario"
        return 0
    else
        echo "FAIL: $scenario"
        return 1
    fi
}

function when {
    local condition="$1"
    shift
    "$@"
}

function given {
    local pre_condition="$1"
    shift
    "$@"
}

function scenario {
    local scenario="$1"
    shift
    "$@"
}

When you write:

scenario "Some test case" \
    given "Something always true" \
    when "Something you want to test" \
    then_expect "Some outcome"

Then it executes:

some_test "Something always true" "Something you want to test" "Some outcome"

And prints one of:

PASS Some test case
FAIL Some test case

A real example at https://github.com/binkley/shell/tree/master/testing-bash.