6.2. The print program

6.2.1. Printing selected fields

The print command in awk outputs selected data from the input file.

When awk reads a line of a file, it divides the line in fields based on the specified input field separator, FS, which is an awk variable (see Section 6.3.2). This variable is predefined to be one or more spaces or tabs.

The variables $1, $2, $3, ..., $N hold the values of the first, second, third until the last field of an input line. The variable $0 (zero) holds the value of the entire line. This is depicted in the image below, where we see six colums in the output of the df command:

Figure 6-1. Fields in awk

In the output of ls -l, there are 9 columns. The print statement uses these fields as follows:

kelly@octarine ~/test> ls -l | awk '{ print $5 $9 }'

kelly@octarine ~/test>

This command printed the fifth column of a long file listing, which contains the file size, and the last column, the name of the file. This output is not very readable unless you use the official way of referring to columns, which is to separate the ones that you want to print with a comma. In that case, the default output separater character, usually a space, will be put in between each output field.

NoteLocal configuration

Note that the configuration of the output of the ls -l command might be different on your system. Display of time and date is dependent on your locale setting.

6.2.2. Formatting fields

Without formatting, using only the output separator, the output looks rather poor. Inserting a couple of tabs and a string to indicate what output this is will make it look a lot better:

kelly@octarine ~/test> ls -ldh * | grep -v total | \ 
awk '{ print "Size is " $5 " bytes for " $9 }'
Size is 160 bytes for orig
Size is 121 bytes for script.sed
Size is 120 bytes for temp_file
Size is 126 bytes for test
Size is 120 bytes for twolines
Size is 441 bytes for txt2html.sh

kelly@octarine ~/test>

Note the use of the backslash, which makes long input continue on the next line without the shell interpreting this as a separate command. While your command line input can be of virtually unlimited length, your monitor is not, and printed paper certainly isn't. Using the backslash also allows for copying and pasting of the above lines into a terminal window.

The -h option to ls is used for supplying humanly readable size formats for bigger files. The output of a long listing displaying the total amount of blocks in the directory is given when a directory is the argument. This line is useless to us, so we add an asterisk. We also add the -d option for the same reason, in case asterisk expands to a directory.

The backslash in this example marks the continuation of a line. See Section 3.3.2.

You can take out any number of columns and even reverse the order. In the example below this is demonstrated for showing the most critical partitions:

kelly@octarine ~> df -h | sort -rnk 5 | head -3 | \ 
awk '{ print "Partition " $6 "\t: " $5 " full!" }'
Partition /var  : 86% full!
Partition /usr  : 85% full!
Partition /home : 70% full!

kelly@octarine ~>

The table below gives an overview of special formatting characters:

Table 6-1. Formatting characters for gawk

\aBell character
\nNewline character

Quotes, dollar signs and other meta-characters should be escaped with a backslash.

6.2.3. The print command and regular expressions

A regular expression can be used as a pattern by enclosing it in slashes. The regular expression is then tested against the entire text of each record. The syntax is as follows:

awk 'EXPRESSION { PROGRAM }' file(s)

The following example displays only local disk device information, networked file systems are not shown:

kelly is in ~> df -h | awk '/dev\/hd/ { print $6 "\t: " $5 }'
/       : 46%
/boot   : 10%
/opt    : 84%
/usr    : 97%
/var    : 73%
/.vol1  : 8%

kelly is in ~>

Slashes need to be escaped, because they have a special meaning to the awk program.

Below another example where we search the /etc directory for files ending in ".conf" and starting with either "a" or "x", using extended regular expressions:

kelly is in /etc> ls -l | awk '/\<(a|x).*\.conf$/ { print $9 }'

kelly is in /etc>

This example illustrates the special meaning of the dot in regular expressions: the first one indicates that we want to search for any character after the first search string, the second is escaped because it is part of a string to find (the end of the file name).

6.2.4. Special patterns

In order to precede output with comments, use the BEGIN statement:

kelly is in /etc> ls -l | \
awk 'BEGIN { print "Files found:\n" } /\<[a|x].*\.conf$/ { print $9 }'
Files found:

kelly is in /etc>

The END statement can be added for inserting text after the entire input is processed:

kelly is in /etc> ls -l | \
awk '/\<[a|x].*\.conf$/ { print $9 } END { print \
"Can I do anything else for you, mistress?" }'
Can I do anything else for you, mistress?

kelly is in /etc>

6.2.5. Gawk scripts

As commands tend to get a little longer, you might want to put them in a script, so they are reusable. An awk script contains awk statements defining patterns and actions.

As an illustration, we will build a report that displays our most loaded partitions. See Section 6.2.2.

kelly is in ~> cat diskrep.awk
/\<[8|9][0-9]%/ { print "Partition " $6 "\t: " $5 " full!" }
END { print "*** Give money for new disks URGENTLY! ***" }

kelly is in ~> df -h | awk -f diskrep.awk
Partition /usr  : 97% full!
*** Give money for new disks URGENTLY! ***

kelly is in ~>

awk first prints a begin message, then formats all the lines that contain an eight or a nine at the beginning of a word, followed by one other number and a percentage sign. An end message is added.

NoteSyntax highlighting

Awk is a programming language. Its syntax is recognized by most editors that can do syntax highlighting for other languages, such as C, Bash, HTML, etc.