tr(1) -cs A-Za-z ' ' | tr(1) A-Z a-z | sort(1) | uniq(1) -c | sort(1) -rn | sed(1) ${1}q
translate or delete characters
-c, -C, --complement
       use the complement of SET1
-s, --squeeze-repeats
       replace  each  input  sequence  of  a  repeated  character  that  is  listed in SET1 with a single
       occurrence of that character
tr [OPTION]... SET1 [SET2]

Translate, squeeze, and/or delete characters from standard input, writing to standard output.

SETs are specified as strings of characters.  Most represent themselves.  Interpreted sequences are:

\NNN   character with octal value NNN (1 to 3 octal digits)

\\     backslash

\a     audible BEL

\b     backspace

\f     form feed

\n     new line

\r     return

\t     horizontal tab

\v     vertical tab

CHAR1-CHAR2
       all characters from CHAR1 to CHAR2 in ascending order

[CHAR*]
       in SET2, copies of CHAR until length of SET1

[CHAR*REPEAT]
       REPEAT copies of CHAR, REPEAT octal if starting with 0

[:alnum:]
       all letters and digits

[:alpha:]
       all letters

[:blank:]
       all horizontal whitespace

[:cntrl:]
       all control characters

[:digit:]
       all digits

[:graph:]
       all printable characters, not including space

[:lower:]
       all lower case letters

[:print:]
       all printable characters, including space

[:punct:]
       all punctuation characters

[:space:]
       all horizontal or vertical whitespace

[:upper:]
       all upper case letters

[:xdigit:]
       all hexadecimal digits

[=CHAR=]
       all characters which are equivalent to CHAR
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).
sort lines of text files
report or omit repeated lines
-c, --count
       prefix lines by the number of occurrences
-r, --reverse
       reverse the result of comparisons
-n, --numeric-sort
       compare according to string numerical value
stream editor for filtering and transforming text
If no -e, --expression, -f, or --file option is given, then the first non-option argument is taken as the
sed script to interpret.  All remaining arguments are names  of  input  files;  if  no  input  files  are
specified, then the standard input is read.
source manpages: trtrsortuniqsortsed