Gawk/Very-Simple
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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
(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 '/li/ { print $0 }' mail-list -| Amelia 555-5553 [email protected] F -| Broderick 555-0542 [email protected] R -| Julie 555-6699 [email protected] F -| Samuel 555-3430 [email protected] 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 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 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 toBEGIN
.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.
Next: Two Rules, Previous: Sample Data Files, Up: Getting Started [Contents][Index]