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(gawk.info)Very Simple

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Some Simple Examples
====================

   The following command runs a simple `awk' program that searches the
input file `BBS-list' for the character string `foo'.  (A string of
characters is usually called a "string".  The term "string" is based on
similar usage in English, such as "a string of pearls," or, "a string
of cars in a train."):

     awk '/foo/ { print $0 }' BBS-list

When lines containing `foo' are found, they are printed because
`print $0' means print the current line.  (Just `print' by itself means
the same thing, so we could have written that instead.)

   You will notice that slashes (`/') surround the string `foo' in the
`awk' program.  The slashes indicate that `foo' is the pattern to
search for.  This type of pattern is called a "regular expression",
which is covered in more detail later (*note Regular Expressions:
Regexp.).  The pattern is allowed to match parts of words.  There are
single quotes around the `awk' program so that the shell won't
interpret any of it as special shell characters.

   Here is what this program prints:

     $ awk '/foo/ { print $0 }' BBS-list
     -| fooey        555-1234     2400/1200/300     B
     -| foot         555-6699     1200/300          B
     -| macfoo       555-6480     1200/300          A
     -| sabafoo      555-2127     1200/300          C

   In an `awk' rule, either the pattern or the action can be omitted,
but not both.  If the pattern is omitted, then the action is performed
for _every_ input line.  If the action is omitted, the default action
is to print all lines that match the pattern.

   Thus, we could leave out the action (the `print' statement and the
curly braces) in the above example and the result would be the same: all
lines matching the pattern `foo' are printed.  By comparison, omitting
the `print' statement but retaining the curly braces makes an empty
action that does nothing (i.e., no lines are printed).

   Many practical `awk' programs are just a line or two.  Following is a
collection of useful, short programs to get you started.  Some of these
programs contain constructs that haven't been covered yet. (The
description of the program will give you a good idea of what is going
on, but please read the rest of the Info file to become an `awk'
expert!)  Most of the examples use a data file named `data'.  This is
just a placeholder; if you use these programs yourself, substitute your
own file names for `data'.  For future reference, note that there is
often more than one way to do things in `awk'.  At some point, you may
want to look back at these examples and see if you can come up with
different ways to do the same things shown here:

   * Print the length of the longest input line:

          awk '{ if (length($0) > max) max = length($0) }
               END { print max }' data

   * Print every line that is longer than 80 characters:

          awk 'length($0) > 80' data

     The sole rule has a relational expression as its pattern and it
     has no action--so the default action, printing the record, is used.

   * Print the length of the longest line in `data':

          expand data | awk '{ if (x < length()) x = length() }
                        END { print "maximum line length is " x }'

     The input is processed by the `expand' utility to change tabs into
     spaces, so the widths compared are actually the right-margin
     columns.

   * Print every line that has at least one field:

          awk 'NF > 0' data

     This is an easy way to delete blank lines from a file (or rather,
     to create a new file similar to the old file but from which the
     blank lines have been removed).

   * Print seven random numbers from 0 to 100, inclusive:

          awk 'BEGIN { for (i = 1; i <= 7; i++)
                           print int(101 * rand()) }'

   * Print the total number of bytes used by FILES:

          ls -l FILES | awk '{ x += $5 }
                            END { print "total bytes: " x }'

   * Print the total number of kilobytes used by FILES:

          ls -l FILES | awk '{ x += $5 }
             END { print "total K-bytes: " (x + 1023)/1024 }'

   * Print a sorted list of the login names of all users:

          awk -F: '{ print $1 }' /etc/passwd | sort

   * Count lines in a file:

          awk 'END { print NR }' data

   * Print the even-numbered lines in the data file:

          awk 'NR % 2 == 0' data

     If you use the expression `NR % 2 == 1' instead, it would print
     the odd-numbered lines.