Why does grep not show colors despite –color=always

colorsgrepregular expression

Works:

echo '\example' | grep --color=always -i '.example'

Works:

echo '\example' | grep --color=always -i '\\example'

Matches, but missing colors:

echo '\example' | grep --color=always -i '\example'

Best Answer

Finding the discrepancy

I spent some time poking through the source code for grep and narrowed down the problem a bit.

The main issue lies in the function print_line_middle. See this loop construct:

for (cur = beg;
     (cur < lim && ((match_offset = execute (compiled_pattern, beg, lim - beg,
                                             &match_size, cur)) != (size_t) -1));
     cur = b + match_size)
{

The discrepancy

The issue is that for example #3 above, the match_offset = execute(...) call is equal to -1. This result influences when the printing is done and thereby what colors are used.

Is this a bug?

I have no idea :) but feel free to send the devs an email. From the README:

Send bug reports to bug-grep@gnu.org.

Related Solutions

Grep –color Adds ANSI Code ESC[K – How to Manage Displayed Text

You can change this behavior by setting the GREP_COLORS environment variable:

export GREP_COLORS=ne
echo -e "ab\rc" | grep --color=always "c"

From the grep man page:

          ne     Boolean  value  that prevents clearing to the end of line
                 using Erase in Line (EL) to Right  (\33[K)  each  time  a
                 colorized  item  ends.   This  is  needed on terminals on
                 which EL is not supported.  It  is  otherwise  useful  on
                 terminals  for  which  the back_color_erase (bce) boolean
                 terminfo capability  does  not  apply,  when  the  chosen
                 highlight colors do not affect the background, or when EL
                 is too slow or causes too much flicker.  The  default  is
                 false (i.e., the capability is omitted).

It's done in the first place to set the background of the rest of the line to the correct color, in case it was changed earlier (though by default it isn't; somebody might set it up to do so in their own settings).

You may also want to play with the other options that can be set in GREP_COLORS; see the man page for full details.

Ubuntu – Is GREP_OPTIONS= –color=always ignored

If grep o produces color output, then either grep is an alias to grep --color=auto or grep --color=always (or possibly more options), or GREP_OPTIONS is set to a value that contains --color=auto or --color=always. Since $GREP_OPTIONS is empty, it must be the alias.

Since grep o | less -R doesn't show colors, the alias must be to grep --color=auto (a sensible choice). With the alias, the grep command always receives the --color option on the command line, and this takes precedence over the environment variable.

If you want to use the environment variable, remove the alias definition from your ~/.bashrc, or for one session run unalias grep. You can replace alias grep='grep --color=auto' by export GREP_OPTIONS='--color=auto': they have essentially the same meaning, except that:

setting GREP_OPTIONS to a different value only overrides the latter;
the alias only kicks in when you run grep from an interactive shell, whereas setting GREP_OPTIONS also applies when grep is run from scripts and other applications.

Never put --color=always or most other options in GREP_OPTIONS: it would break many programs that parse the output of grep. --color=auto is about the only safe option to put in GREP_OPTIONS. For anything else, use the alias. Future versions of GNU grep will drop support for the option for this reason.

Note that the alias definition goes into ~/.bashrc (it's a shell setting), whereas the environment variable definition goes into ~/.profile (it's a session setting). See Is there a ".bashrc" equivalent file read by all shells?

If you want to run the unaliased command just once, run \grep instead of grep (quoting any part of the name bypasses the alias lookup).