Adding FAQ entry: shell built-in (echo/printf/etc.)

[Assaf Gordon]

Hello,

What do you think about adding a FAQ entry about confusing shell builtin 
with coreutils' programs?

Will apply to echo,printf,test,[,kill.

Seems like common enough topic:
http://lists.gnu.org/archive/html/bug-coreutils/2013-03/msg00016.html
http://lists.gnu.org/archive/html/bug-coreutils/2013-05/msg00013.html
http://lists.gnu.org/archive/html/bug-coreutils/2014-05/msg00099.html
http://lists.gnu.org/archive/html/bug-coreutils/2015-08/msg00030.html

Something like this:
====
Q. echo/test/printf/test/[/kill do not work as documented.
Commonly, one of these doesn't work (yet expected to work):

    echo --help
    echo --version
    echo -e '\x45'
    printd --version
    printf '\u263a\n'
    kill --help

A. These commands are often implemented as shell builtin functions,
supporting less features than GNU coreutils' versions.

Use 'type' to determine if the command is a shell builtin:

    $ type printf
    printf is a shell builtin

versus:

    $ type printf
    printf is /usr/bin/printf

To use the external version of these programs, prefix with 'env' or 
specify a full path.

shell builtin functions on do not support --version:

    $ echo --version
    --version
    $ printf --version
    bash: printf: --: invalid option
    printf: usage: printf [-v var] format [arguments]


while GNU coreutils programs do support --version (and other features
mentioned in the documentation):

    $ env echo --version | head -n1
    echo (GNU coreutils) 8.21
    $ env printf --version | head -n1
    printf (GNU coreutils) 8.21
    $ /bin/echo --version | head -n1
    echo (GNU coreutils) 8.21
    $ env echo -e '==\x3D=='
    =====
    $ env printf '\u263a\n'
    [prints unicode smily face, if supported by your terminal]

These features often extends POSIX, and are not portable.
To restore strict POSIX compliance, use POSIXLY_CORRECT environment 
variable as explained by Eric Blake:
http://lists.gnu.org/archive/html/bug-coreutils/2013-03/msg00016.html
====


regards,
 - assaf