[FFmpeg-devel] portable shell programming

[Jethro Walters]

Diego Biurrun wrote:
> Since the topic cropped up after Stefano committed some bashisms, I
> thought this might be helpful.
> 
> I recently wrote a quick guide to portable shell programming at work,
> I'm pasting it here.  It's neither complete nor exhaustive, but it's a
> start and it should help avoid the most glaring mistakes.  Further
> comments and additions are welcome.
> 
> 
> But first maybe I should explain what the point of this whole exercise
> is.  Why bother fixing bashism when bash is ubiquitous free software?
> It's true that bash is available everywhere and yes, it is the default
> shell on most modern systems.  Or rather, it is the default
> *interactive* shell on most modern systems.
> 
> bash has some drawbacks.  It is
> 
> - slow,
> - bloated,
> - has nonstandard syntax extensions.
> 
> Nowadays this tends to make people yawn and say that they don't care if
> program XYZ uses up another 0.1% of their CPU cycles and/or another 0.1%
> of their memory.
> 
> However, FFmpeg is expected to be portable and work on small embedded
> systems.
> 
> Even for fullblown desktop machines performance is important.  Ubuntu
> switched from bash to dash as /bin/sh (not default interactive shell)
> and boot speed improved by around 30% IIRC.
> 
> Furthermore, nobody likes it when Microsoft bends and extends standards,
> so you should not let the GNU people get away with it.
> 
> Notice that both Ubuntu and Debian use dash instead of bash nowadays.
> I'm not sure about other environments, but there are more and the number
> is increasing.
> 
> OK, now that you are convinced that writing portable shell scripts is
> the way forward, here is how you go about it:
> 
> 
> * default shell
>   Make sure your system does not use bash as /bin/sh.
> 
> 
> * correct shebang line
>   Add a proper '#!/bin/sh' at the top of your shell scripts.  Don't use
>   /bin/bash there.  Don't leave out the shebang line, behavior is
>   non-deterministic without it.
> 
> 
> * 'source' vs. '.'
>   Use '.' instead of 'source' when including other files in your shell
>   script.  'source' is bash-specific.
> 
> 
> * 'let' and '(( ))'
>   Both are bashisms.  Use arithmetic expansion with '$(( ))' instead.
> 
> 
> * skip the 'function' keyword for function declarations
>   Just use
> 
>     foo() { ....}
> 
>   instead of
> 
>     function foo() { ....}
> 
>   Bash understands both, POSIX only specifies the former, so use that.
> 
> 
> * '==' in test invocations
>   The builtin 'test' command in bash understands '==', but POSIX test
>   does not and the builtin in other shells may or may not understand
>   '=='.  Just use '=' instead.
> 
> 
> * 'echo' with backslash escapes
>   The builtin 'echo' in some shells does not support backslash escapes.
>   Use '/bin/echo' or 'printf' instead if you absolutely need escape
>   sequences.
> 
> 
> * 'read' only works with a variable
>   Write 'read line' instead of 'read', it's that simple.
> 
> 
> * 'getopt' vs. 'getopts'
>   The one with the 's' at the end is bash-specific, prefer the other.
> 
> 
> * some parameter expansion expressions are non-portable
>   An example is '${CMD:1}'.  There are more bash-specific ones that I
>   don't remember offhand.  Use just the POSIX-compatible ones.  See
>   http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_06_02
> 
> 
> * `` vs. $()
>   Both are POSIX-compatible and backticks are even a tad more portable,
>   but I wholeheartedly recommend the $() syntax.  Nesting backticks is
>   possible, but will drive you crazy in a very short amount of time.
> 
> 
> If you want to know if any bashisms crept into your script, try
> 'checkbashisms' from the Debian devscripts package.  It will check
> your scripts for the most common bashisms.  But the most important
> part is checking your scripts with dash or another POSIX-compatible
> non-bash shell.
> 
> The POSIX specification is hosted at
> 
> http://www.opengroup.org/onlinepubs/009695399/
> 
> Refer to the "Shell & Utilities" section, also at
> 
> http://www.opengroup.org/onlinepubs/009695399/idx/xcu.html
> 
> 
> That's it for now.  Further questions and feedback are welcome.
> 
> Diego

As an occasional programmer, I welcome this. /me keeps. :)

Regards,

Jethro Walters