1.3 Some Simple Examples

The following command runs a simple awk program that searches the input file ‘BBS-list’ for the character string ‘foo’ (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 '/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 (see section Regular Expressions). 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 previous example and the result would be the same: awk prints all lines matching the pattern ‘foo’. 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 Web page 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 / 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 use the expression ‘NR % 2 == 1’ instead, the program would print the odd-numbered lines.