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gawk: Very Simple

 1.3 Some Simple Examples
 The following command runs a simple 'awk' program that searches the
 input file 'mail-list' for the character string 'li' (a grouping 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 '/li/ { print $0 }' mail-list
 When lines containing 'li' 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 'li' in the
 'awk' program.  The slashes indicate that 'li' is the pattern to search
 for.  This type of pattern is called a "regular expression", which is
 covered in more detail later (⇒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
    Here is what this program prints:
      $ awk '/li/ { print $0 }' mail-list
      -| Amelia       555-5553    F
      -| Broderick    555-0542 R
      -| Julie        555-6699   F
      -| Samuel       555-3430        A
    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
 braces) in the previous example and the result would be the same: 'awk'
 prints all lines matching the pattern 'li'.  By comparison, omitting the
 'print' statement but retaining the braces makes an empty action that
 does nothing (i.e., no lines are printed).
    Many practical 'awk' programs are just a line or two long.  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 you'll need to 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 every line that is longer than 80 characters:
           awk 'length($0) > 80' data
      The sole rule has a relational expression as its pattern and has no
      action--so it uses the default action, printing the record.
    * Print the length of the longest input line:
           awk '{ if (length($0) > max) max = length($0) }
                END { print max }' data
      The code associated with 'END' executes after all input has been
      read; it's the other side of the coin to 'BEGIN'.
    * Print the length of the longest line in 'data':
           expand data | awk '{ if (x < length($0)) x = length($0) }
                              END { print "maximum line length is " x }'
      This example differs slightly from the previous one: the input is
      processed by the 'expand' utility to change TABs into spaces, so
      the widths compared are actually the right-margin columns, as
      opposed to the number of input characters on each line.
    * 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 / 1024 }'
    * Print a sorted list of the login names of all users:
           awk -F: '{ print $1 }' /etc/passwd | sort
    * Count the 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 used the expression 'NR % 2 == 1' instead, the program would
      print the odd-numbered lines.
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