[ovs-dev] [PATCH 1/2] acinclude: Add support for grep option.

[Ben Pfaff]

On Mon, Oct 16, 2017 at 07:26:44AM -0700, William Tu wrote:
> Allow to pass grep's option to OVS_GREP_IFELSE.
> One use case is to pass '-w' for exact match.
> 
> Signed-off-by: William Tu <u9012063 at gmail.com>

POSIX doesn't mention a -w option, and the Autoconf manual says that it
is not portable in practice.  It also says that \b is not portable in
practice.

Is there another way to accomplish what you want to do?  For example,
how about HAVE_SKB_GSO_UDP[^_]?  Since this is Autoconf, probably it's
necessary to double the [], as: HAVE_SKB_GSO_UDP[[^_]]

(We don't really care that much about portability to everything that
Autoconf supports, so probably we could really use -w or \b in
practice.)

This is what the Autoconf manual says:

'grep'
     Portable scripts can rely on the 'grep' options '-c', '-l', '-n',
     and '-v', but should avoid other options.  For example, don't use
     '-w', as Posix does not require it and Irix 6.5.16m's 'grep' does
     not support it.  Also, portable scripts should not combine '-c'
     with '-l', as Posix does not allow this.

     Some of the options required by Posix are not portable in practice.
     Don't use 'grep -q' to suppress output, because many 'grep'
     implementations (e.g., Solaris) do not support '-q'.  Don't use
     'grep -s' to suppress output either, because Posix says '-s' does
     not suppress output, only some error messages; also, the '-s'
     option of traditional 'grep' behaved like '-q' does in most modern
     implementations.  Instead, redirect the standard output and
     standard error (in case the file doesn't exist) of 'grep' to
     '/dev/null'.  Check the exit status of 'grep' to determine whether
     it found a match.

     The QNX4 implementation fails to count lines with 'grep -c '$'',
     but works with 'grep -c '^''.  Other alternatives for counting
     lines are to use 'sed -n '$='' or 'wc -l'.

     Some traditional 'grep' implementations do not work on long input
     lines.  On AIX the default 'grep' silently truncates long lines on
     the input before matching.

     Also, many implementations do not support multiple regexps with
     '-e': they either reject '-e' entirely (e.g., Solaris) or honor
     only the last pattern (e.g., IRIX 6.5 and NeXT). To work around
     these problems, invoke 'AC_PROG_GREP' and then use '$GREP'.

     Another possible workaround for the multiple '-e' problem is to
     separate the patterns by newlines, for example:

          grep 'foo
          bar' in.txt

     except that this fails with traditional 'grep' implementations and
     with OpenBSD 3.8 'grep'.

     Traditional 'grep' implementations (e.g., Solaris) do not support
     the '-E' or '-F' options.  To work around these problems, invoke
     'AC_PROG_EGREP' and then use '$EGREP', and similarly for
     'AC_PROG_FGREP' and '$FGREP'.  Even if you are willing to require
     support for Posix 'grep', your script should not use both '-E' and
     '-F', since Posix does not allow this combination.

     Portable 'grep' regular expressions should use '\' only to escape
     characters in the string '$()*.0123456789[\^{}'.  For example,
     alternation, '\|', is common but Posix does not require its support
     in basic regular expressions, so it should be avoided in portable
     scripts.  Solaris and HP-UX 'grep' do not support it.  Similarly,
     the following escape sequences should also be avoided: '\<', '\>',
     '\+', '\?', '\`', '\'', '\B', '\b', '\S', '\s', '\W', and '\w'.

     Posix does not specify the behavior of 'grep' on binary files.  An
     example where this matters is using BSD 'grep' to search text that
     includes embedded ANSI escape sequences for colored output to
     terminals ('\033[m' is the sequence to restore normal output); the
     behavior depends on whether input is seekable:

          $ printf 'esc\033[mape\n' > sample
          $ grep . sample
          Binary file sample matches
          $ cat sample | grep .
          escape