Info Catalog grep: Exit Status grep: Invoking

grep: grep Programs

 
 2.4 ‘grep’ Programs
 ===================
 
 ‘grep’ searches the named input files for lines containing a match to
 the given patterns.  By default, ‘grep’ prints the matching lines.  A
 file named ‘-’ stands for standard input.  If no input is specified,
 ‘grep’ searches the working directory ‘.’ if given a command-line option
 specifying recursion; otherwise, ‘grep’ searches standard input.  There
 are four major variants of ‘grep’, controlled by the following options.
 
 ‘-G’
 ‘--basic-regexp’
      Interpret patterns as basic regular expressions (BREs).  This is
      the default.
 
 ‘-E’
 ‘--extended-regexp’
      Interpret patterns as extended regular expressions (EREs).  (‘-E’
      is specified by POSIX.)
 
 ‘-F’
 ‘--fixed-strings’
      Interpret patterns as fixed strings, not regular expressions.
      (‘-F’ is specified by POSIX.)
 
 ‘-P’
 ‘--perl-regexp’
      Interpret patterns as Perl-compatible regular expressions (PCREs).
      PCRE support is here to stay, but consider this option experimental
      when combined with the ‘-z’ (‘--null-data’) option, and note that
      ‘grep -P’ may warn of unimplemented features.  ⇒Other
      Options.
 
      For documentation, refer to <https://www.pcre.org/>, with these
      caveats:
         • ‘\d’ matches only the ten ASCII digits (and ‘\D’ matches the
           complement), regardless of locale.  Use ‘\p{Nd}’ to also match
           non-ASCII digits.  (The behavior of ‘\d’ and ‘\D’ is
           unspecified after in-regexp directives like ‘(?aD)’.)
 
         • Although PCRE tracks the syntax and semantics of Perl's
           regular expressions, the match is not always exact.  For
           example, Perl evolves and a Perl installation may predate or
           postdate the PCRE2 installation on the same host, or their
           Unicode versions may differ, or Perl and PCRE2 may disagree
           about an obscure construct.
 
         • By default, ‘grep’ applies each regexp to a line at a time, so
           the ‘(?s)’ directive (making ‘.’ match line breaks) is
           generally ineffective.  However, with ‘-z’ (‘--null-data’) it
           can work:
                $ printf 'a\nb\n' |grep -zP '(?s)a.b'
                a
                b
           But beware: with the ‘-z’ (‘--null-data’) and a file
           containing no NUL byte, grep must read the entire file into
           memory before processing any of it.  Thus, it will exhaust
           memory and fail for some large files.
 

Info Catalog grep: Exit Status grep: Invoking