search for files in a directory hierarchy

find [-H] [-L] [-P] [-D debugopts] [-Olevel] [path...] [expression]

-type c
       File is of type c:

       b      block (buffered) special

       c      character (unbuffered) special

       d      directory

       p      named pipe (FIFO)

       f      regular file

       l      symbolic link; this is never true if the -L option or the  -follow  option  is  in  effect,
              unless the symbolic link is broken.  If you want to search for symbolic links when -L is in
              effect, use -xtype.

       s      socket

       D      door (Solaris)

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).

how to execute the Perl interpreter

-n   causes Perl to assume the following loop around your program, which makes it iterate over filename
     arguments somewhat like sed -n or awk:

              LINE:
                while (<>) {
                    ...             # your program goes here
                }

            Note that the lines are not printed by default.  See -p to have lines printed.  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.

            Also note that "<>" passes command line arguments to "open" in perlfunc, which doesn't necessarily
            interpret them as file names.  See  perlop for possible security implications.

            Here is an efficient way to delete all files that haven't been modified for at least a week:

                find . -mtime +7 -print | perl -nle unlink

            This is faster than using the -exec switch of find because you don't have to start a process on
            every filename found.  It does suffer from the bug of mishandling newlines in pathnames, which you
            can fix if you follow the example under -0.

            "BEGIN" and "END" blocks may be used to capture control before or after the implicit program 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.

sort lines of text files

report or filter out repeated lines in a file

-c     Precede each output line with a count of the number of times the line occurred in the input.

-n, --numeric-sort
       compare according to string numerical value