How to Write AWK Commands and Scripts

The awk command is a powerful method for processing or analyzing text files. In particular, it analyzes data files that are organized by lines (rows) and columns. Simple awk commands can be run from the command line. More complex tasks should be written as awk programs (called awk scripts) to a file.

The basic format of an awk command looks like this:

awk 'pattern {action}' input-file > output-file

This means: take each line of the input file; if the line contains the pattern, apply the action to the line and write the resulting line to the output file. If the pattern is omitted, the action is applied to all lines. For example:

awk '{ print $5 }' table1.txt > output1.txt

The above statement takes the element of the 5th column of each line and writes it as a line in the output file "output.txt." The variable '$4' refers to the fourth column. Similarly, you can access the first, second, and third column, with $1, $2, $3, etc. By default, columns are assumed to be separated by spaces or tabs (so-called white space). So, if the input file "table1.txt" contains these lines:

1, Justin Timberlake, Title 545, Price $7.30
2, Taylor Swift, Title 723, Price $7.90
3, Mick Jagger, Title 610, Price $7.90
4, Lady Gaga, Title 118, Price $7.30
5, Johnny Cash, Title 482, Price $6.50
6, Elvis Presley, Title 335, Price $7.30
7, John Lennon, Title 271, Price $7.90
8, Michael Jackson, Title 373, Price $5.50

Then the command would write the following lines to the output file "output1.txt":

545,
723,
610,
118,
482,
335,
271,
373,

If the column separator is something other than spaces or tabs, such as a comma, you can specify that in the awk statement as follows:

awk -F, '{ print $3 }' table1.txt > output1.txt

This will select the element from column 3 of each line if the columns are considered to be separated by a comma. Therefore the output, in this case, would be:

 Title 545
 Title 723
 Title 610
 Title 118
 Title 482
 Title 335
 Title 271
 Title 373

The list of statements inside the curly brackets ('{','}') is called a block. If you put a conditional expression in front of a block, the statement inside the block will be executed only if the condition is true.

awk '$7=="\$7.30" { print $3 }' table1.txt

In this case, the condition is $7=="\$7.30", which means that the element in column 7 is equal to $7.30. The backslash in front of the dollar sign is used to prevent the system from interpreting $7 as a variable and instead take the dollar sign literally.

So this awk statement prints out the element at the 3rd column of each line that has a "$7.30" in column 7.

You can also use regular expressions as the condition. For example:

awk '/30/ { print $3 }' table1.txt

The string between the two slashes ('/') is the regular expression. In this case, it is just the string "30." This means if a line contains the string "30", the system prints out the element at the 3rd column of that line. The output in the above example would be:

Timberlake,
Gaga,
Presley,

If the table elements are numbers, awk can run calculations on them as in this example:

awk '{ print ($2 * $3) + $7 }'

Besides the variables that access elements of the current row ($1, $2, etc.), there is the variable $0 which refers to the complete row (line), and the variable NF which holds to the number of fields.

You can also define new variables as in this example:

awk '{ sum=0; for (col=1; col<=NF; col++) sum += $col; print sum; }'

This computes and prints the sum of all the elements of each row.