Re: [PATCH v2 001/142] check-builtins.sh: use the $( ... ) construct for command substitution

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 25, 2014, 20:36 UTC
Message-ID: <xmqqeh1qkobn.fsf@gitster.dls.corp.google.com>
In-Reply-To: <xmqqtxamkq4b.fsf@gitster.dls.corp.google.com>

Junio C Hamano <gitster@pobox.com> writes:

Show 23 quoted lines

> Elia Pinto <gitter.spiros@gmail.com> writes:
>
>> The Git CodingGuidelines prefer the $( ... ) construct for command
>> substitution instead of using the back-quotes, or grave accents (`..`).
>>
>> The backquoted form is the historical method for command substitution,
>> and is supported by POSIX. However, all but the simplest uses become
>> complicated quickly. In particular, embedded command substitutions
>> and/or the use of double quotes require careful escaping with the backslash
>> character. Because of this the POSIX shell adopted the $(…) feature from
>> the Korn shell.
>>
>> The patch was generated by the simple script
>>
>> for _f in $(find . -name "*.sh")
>> do
>>   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
>> done
>
> "and then carefully proofread" is sorely needed here.
>
> What is that non-breaking space doing at the beginning of an
> indented line, or is it just my environment, by the way?

I've reworded the above like so:

    check-builtins.sh: use the $(...) construct for command substitution
    
    The Git CodingGuidelines prefer the $(...) construct for command
    substitution instead of using the backquotes, or grave accents
    (`...`).
    
    The backquoted form is the historical method for command
    substitution, and is supported by POSIX.  However, all but the
    simplest uses become complicated quickly.  In particular, embedded
    command substitutions and/or the use of double quotes require
    careful escaping with the backslash character.
    
    The patch was generated by:
    
        for _f in $(find . -name "*.sh")
        do
          sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
        done
    
    and then carefully proof-read.
    
    Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
    Signed-off-by: Junio C Hamano <gitster@pobox.com>

I looked at [*1*] to pick between "back-quote" and "backquote"; this is a related tangent, but in that page, you will find this gem:

    Within the backquoted style of command substitution, <backslash>
    shall retain its literal meaning, except when followed by: '$',
    '`', or <backslash>.

Stated another way, `` => $() conversion will make backslash inside to bahave differently, and we would need to be careful when doing such a change.

I've looked at and queued 001 and 002.

[Reference]

*1* http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_03