The Unbearable Lightness: Extending the Bash shell
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This document summarizes challenges and limitations of the Bash shell and discusses various attempts to address them through frameworks, libraries, and paradigms like object orientation, functional programming, and inversion of control. It highlights issues like variable scoping, lack of exceptions, sorting, parsing, binary data handling, and debugging as well as efforts like bashlets, bashinator, bash manager, and oobash to improve modularity, reuse, and robustness of Bash scripts.
![Multidimensional
arrays
$ array=('d1=(v1 v2 v3)' 'd2=(v1 v2 v3)')
$ for elt in "${array[@]}";do eval $elt;done
$ echo "d1 ${#d1[@]} ${d1[@]}"
$ echo "d2 ${#d2[@]} ${d2[@]}"](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-12-320.jpg)
![Sorting
$ array=(2 1 0)
$ IFS=$'n' sorted=($(sort <<<"${array[*]}"))
$ unset IFS
$ echo ${sorted[*]}
0 1 2](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-13-320.jpg)
![Function scope
$ shopt -s extdebug
$ function foo() { echo $qux ; echo ${BASH_ARGV[0]} ; }
$ function bar() { local qux=thud ; foo ; }
$bar baz
thud
baz](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-18-320.jpg)
![Function collapsing
chatter() {
if [[ $verbose ]]; then
chatter() {
echo "$@"
}
chatter "$@"
else
chatter() {
:
}
fi
}](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-19-320.jpg)
["']$s$|1$fs2$fs3|p"
-e "s|^($s)($w)$s:$s(.*)$s$|1$fs2$fs3|p"](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-24-320.jpg)
![bashinator
function createDirectory() {
local directory=${1}
if [[ -z "${directory}" ]]; then
__msg err "argument 1 (directory) missing"
return 2
fi
__msg debug "directory: ${directory}"
if ! mkdir -p "${directory}" >> "$_{L}" 2>&1; then
__msg err "failed to create directory '${directory}'"
return 2
fi
return 0
}](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-39-320.jpg)
![bash automated
testing system
@test "addition using bc" {
result="$(echo 2+2 | bc)"
[ "$result" -eq 4 ]
}
@test "addition using dc" {
result="$(echo 2 2+p | dc)"
[ "$result" -eq 4 ]
}](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-40-320.jpg)
![mbfl
mbfl_argv_all_files ||
exit_because_wrong_command_line_arguments
for item in "${ARGV[@]}"
do
test "${script_option_AUTO}" = yes && {
size=$(mbfl_file_get_size "${item}")
...](https://image.slidesharecdn.com/linux-day-2016-171004161251/85/The-Unbearable-Lightness-Extending-the-Bash-shell-47-320.jpg)
The Unbearable Lightness: Extending the Bash shell
- 2. The Unbearable Lightness Extending the Bash shell
- 3. I have a natural revulsion to any operating system that shows so little planning as to have named all of its commands after digestive noises (awk, grep, fsck, nroff). Source: The UNIX HATERS Handbook
- 4. Shellshocks
- 5. CVE-2014-6271 env x='() { :;}; echo vulnerable' bash -c "echo this is a test"
- 6. CVE-2014-7169 env X='() { (a)=>' bash -c "echo date"; cat echo
- 7. Weaknesses
- 9. Speed It's too big and too slow. Source: man bash
- 10. Everything is text egrep '^To:|^Cc:' /var/spool/mail/$USER | cut -c5- | awk '{ for (i = 1; i <= NF; i++) print $i }' | sed 's/,//g' | grep -v $USER | sort | uniq
- 11. Floating point $ echo $((3/2)) 1
- 12. Multidimensional arrays $ array=('d1=(v1 v2 v3)' 'd2=(v1 v2 v3)') $ for elt in "${array[@]}";do eval $elt;done $ echo "d1 ${#d1[@]} ${d1[@]}" $ echo "d2 ${#d2[@]} ${d2[@]}"
- 13. Sorting $ array=(2 1 0) $ IFS=$'n' sorted=($(sort <<<"${array[*]}")) $ unset IFS $ echo ${sorted[*]} 0 1 2
- 14. Variable typing $ declare -i x $ x=foo $ echo $x 0
- 15. Variable scope Local vs Global vs Environment
- 16. Return values Bash functions don't return anything (except numbers in the range 0-255); they only produce output streams
- 17. Passing by reference function swap() { eval "$1=${!2} $2=${!1}" } $ x=a y=b $ swap x y b a
- 18. Function scope $ shopt -s extdebug $ function foo() { echo $qux ; echo ${BASH_ARGV[0]} ; } $ function bar() { local qux=thud ; foo ; } $bar baz thud baz
- 19. Function collapsing chatter() { if [[ $verbose ]]; then chatter() { echo "$@" } chatter "$@" else chatter() { : } fi }
- 20. No closures No first-order functions, no lambdas, ...
- 21. Try/catch trap
- 24. Parsing sed -ne "s|^($s):|1|" -e "s|^($s)($w)$s:$s["'](.*)["']$s$|1$fs2$fs3|p" -e "s|^($s)($w)$s:$s(.*)$s$|1$fs2$fs3|p"
- 25. Binary data dd if=/dev/brain of=/dev/null
- 26. Paradigms OO? Functional? Flow-driven? Reflective?
- 27. Security Did you say dropping permissions?
- 29. Debugging set -x
- 30. Subshell Dilemma $ c=0; ls * | while read i; do ((c++)); done; echo $c 0
- 32. Portability It is easier to port a shell than a shell script. Source: Larry Wall
- 34. DRY Don't write scripts, write libraries
- 35. DevOps Python/Ruby/Scala/... are not ubiquitous
- 36. Legacy code
- 37. Attempts
- 39. bashinator function createDirectory() { local directory=${1} if [[ -z "${directory}" ]]; then __msg err "argument 1 (directory) missing" return 2 fi __msg debug "directory: ${directory}" if ! mkdir -p "${directory}" >> "$_{L}" 2>&1; then __msg err "failed to create directory '${directory}'" return 2 fi return 0 }
- 40. bash automated testing system @test "addition using bc" { result="$(echo 2+2 | bc)" [ "$result" -eq 4 ] } @test "addition using dc" { result="$(echo 2 2+p | dc)" [ "$result" -eq 4 ] }
- 41. bash infinity import util/namedParameters util/class class:Human() { public string name public integer height public array eaten Human.__getter__() { echo "I'm a human called $(this name), $(this height) cm tall." } ... }
- 42. bash manager hello: p1=World! $ ./bash_manager.sh -h /tmp/helloHome Hello World!
- 43. bash toolbox alias fail='printf "${BASH_SOURCE##*/}: ${FU ... alias debug='printf "${BASH_SOURCE##*/}: ${FU ... alias caller='nm=$(builtin caller 0); nm=${nm% ...
- 44. bashworks Inversion of control: the overall program's flow of control is not dictated by the caller, but by the framework. Polite functions: Generic reuseable functions usually take a module name string argument.
- 46. log4sh # set alternative 'nc' command log4sh_setAlternative nc /bin/nc # add and configure a SyslogAppender that logs to a remote host logger_addAppender mySyslog appender_setType mySyslog SyslogAppender appender_syslog_setFacility mySyslog local4 appender_syslog_setHost mySyslog somehost appender_activateOptions mySyslog # say Hello to the world logger_info 'Hello, world'
- 47. mbfl mbfl_argv_all_files || exit_because_wrong_command_line_arguments for item in "${ARGV[@]}" do test "${script_option_AUTO}" = yes && { size=$(mbfl_file_get_size "${item}") ...
- 48. oobash ## class Dialog Dialog=( # Explicit definition is needed to make inheritance possible. function __new__ = Dialog::__new__ function __delete__ = Dialog::__delete__ )
- 49. ticktick . ticktick.sh function printEmployees() { for employee in ``people.Engineering.items()``; do printf " - %sn" ${!employee} done }
- 50. Bashlets
- 51. Modular source bashlet datatype/version bash$$ version sort 1.44 1.4 1.4.4 1.4.4a 1.4-1234
- 52. Git integration
- 53. General-purpose IPC, math, network, OS-abstraction, parsers, UX, ...
- 54. Extensible Plugins for web protocols, databases, RCS, ...
- 56. Work in progress
- 57. Colophon vim, Hovercraft!, CentOS Linux 7.2