git diff(1) --name-only HEAD | grep(1) "\.m" | xargs(1) egrep(1) -s --with-filename --line-number --only-matching "$KEYWORDS.*[^\/]*$" | perl(1) -p -e "s/($KEYWORDS)/ warning: Are you sure you want to use \$1?\nTo remove this warning, append a comment at the end of this line \$1/"
Show changes between commits, commit and working tree, etc
--name-only
    Show only names of changed files.
Show changes between the working tree and the index or a tree, changes between the index and a tree,
changes between two trees, or changes between two files on disk.
Pipelines
    A  pipeline is a sequence of one or more commands separated by one of the control operators | or |&.  The
    format for a pipeline is:

           [time [-p]] [ ! ] command [ [||&] command2 ... ]

    The standard output of command is connected  via  a  pipe  to  the  standard  input  of  command2.   This
    connection  is performed before any redirections specified by the command (see REDIRECTION below).  If |&
    is used, the standard error of command is connected to command2's standard input through the pipe; it  is
    shorthand  for  2>&1  |.   This  implicit  redirection  of  the  standard  error  is  performed after any
    redirections specified by the command.

    The return status of a pipeline is the exit status of the last command, unless  the  pipefail  option  is
    enabled.   If  pipefail  is  enabled,  the  pipeline's return status is the value of the last (rightmost)
    command to exit with a non-zero status, or zero if all commands exit successfully.  If the reserved  word
    !   precedes  a  pipeline, the exit status of that pipeline is the logical negation of the exit status as
    described above.  The shell waits for all commands in the pipeline to terminate before returning a value.

    If the time reserved word precedes a pipeline, the elapsed as well as user and system  time  consumed  by
    its execution are reported when the pipeline terminates.  The -p option changes the output format to that
    specified by POSIX.  When the shell is in posix mode, it does not recognize time as a  reserved  word  if
    the  next  token begins with a `-'.  The TIMEFORMAT variable may be set to a format string that specifies
    how the timing information should be displayed; see the description of TIMEFORMAT under  Shell  Variables
    below.

    When the shell is in posix mode, time may be followed by a newline.  In this case, the shell displays the
    total user and system time consumed by the shell and its children.  The TIMEFORMAT variable may  be  used
    to specify the format of the time information.

    Each command in a pipeline is executed as a separate process (i.e., in a subshell).
print lines matching a pattern
grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus
(-) is given as file name) for lines containing a match to the given PATTERN.  By  default,  grep  prints
the matching lines.
build and execute command lines from standard input
-s, --no-messages
       Suppress error messages about nonexistent or unreadable files.  Portability note: unlike GNU grep,
       7th  Edition  Unix  grep  did not conform to POSIX, because it lacked -q and its -s option behaved
       like GNU grep's -q option.  USG-style grep also lacked -q but its -s option behaved like GNU grep.
       Portable  shell  scripts should avoid both -q and -s and should redirect standard and error output
       to /dev/null instead.  (-s is specified by POSIX.)
-H, --with-filename
       Print  the  file  name  for  each  match.  This is the default when there is more than one file to
       search.
-n, --line-number
       Prefix  each  line of output with the 1-based line number within its input file.  (-n is specified
       by POSIX.)
-o, --only-matching
       Print  only  the  matched  (non-empty) parts of a matching line, with each such part on a separate
       output line.
how to execute the Perl interpreter
-p   causes Perl to assume the following loop around your program, which makes it iterate over filename
     arguments somewhat like sed:

       LINE:
         while (<>) {
             ...             # your program goes here
         } continue {
             print or die "-p destination: $!\n";
         }

     If a file named by an argument cannot be opened for some reason, Perl warns you about it, and moves
     on to the next file.  Note that the lines are printed automatically.  An error occurring during
     printing is treated as fatal.  To suppress printing use the -n switch.  A -p overrides a -n switch.

     "BEGIN" and "END" blocks may be used to capture control before or after the implicit loop, just as
     in awk.
-e commandline
     may be used to enter one line of program.  If -e is given, Perl will not look for a filename in the
     argument list.  Multiple -e commands may be given to build up a multi-line script.  Make sure to use
     semicolons where you would in a normal program.
source manpages: git-diffgrepxargsegrepperlrun