awk command
Linux

30 Examples for awk command in text processing

In the previous post, we talked about sed command and we saw many examples of using it in text processing and we saw how it is good in this, but it has some limitations. Sometimes you need a more advanced tool for manipulating data, one that provides a more programming-like environment, giving you more control to modify data in a file. This is where awk command comes in.

The awk command or GNU awk in specific takes stream editing one step further than the sed editor by providing a scripting language instead of just editor commands. With awk scripting language, you can make the following:

  • Define variables to store data.
  • Use string and arithmetic operators.
  • Use control flow, like if-then statements and loops.
  • Generate formatted reports.

Actually, generating formatted reports comes very handy when working with log files that contain maybe millions of lines to output a readable report that you can benefit from.

 

awk command options

The awk command has a basic format as follows:

$ awk options program file

And these are some of the options for the awk command that you will use often:

-F fs     To specify a file separator for the fields in a line.

-f file     To specify a file name to read the program from.

-v var=value     To define a variable and default value used in the awk command.

mf N     To specify the maximum number of fields to process in the data file.

mr N     To specify the maximum record size in the data file.

You can write scripts to read the data from a text line and then manipulate the data and display the result.

Reading the script from the command line

To define an awk script, you have to use opening and closing braces surrounded by single quotation marks like this:

$ awk '{print "Welcome to awk command tutorial"}'

If you press enter, nothing appears!! And this because no filename was defined in the command line.

The awk command gets data from STDIN.

When you type some text and press Enter, the awk command runs the script through the text. Like the sed command, the awk command runs the script on every line of text available in the data.

$ awk '{print "Welcome to awk command tutorial "}'

awk command

If you type anything, it returns the same welcome string we provide.

To terminate the program, we have to send End-of-File (EOF) character. The Ctrl+D key combination sends an EOF character. Looks tricky, don’t panic, the best is yet to come.

Using Data Field Variables

With awk, you can process data in a text file. It does this by automatically assigning a variable to each element in a line. By default, awk uses some built-in variables for each data field found in a line of text:

  • $0 for the entire line of text.
  • $1 for the first field.
  • $2 for the second field.
  • $n for the nth field.

The whitespace character like space or tab is the default separator between fields in awk.

Check this example and see how awk processes it:

$ awk '{print $1}' myfile

awk command variables

This command uses the $1 field variable to display only the first data field for each line of text.

Sometimes the separator in some files is not space nor tab but something else. You can specify it using –F option:

$ awk -F: '{print $1}' /etc/passwd

awk command passwd

This command shows the first data field in the passwd file. Since the /etc/passwd file uses a colon as a field separator, we use it.

Using Multiple Commands

To run multiple commands on the command line, put a semicolon between commands like this:

$ echo "My name is Tom" | awk '{$4="Adam"; print $0}'

awk multiple commands

The first command makes the $4 field variable equals Adam. The second command prints the entire line.

Reading The Script From a File

As with the sed command, the awk command enables you to save your script in a file and refer to it from the command line with the -f option.

Our file contains this script:

{print $1 " home at " $6}

$ awk -F: -f testfile /etc/passwd

awk command read from file

Here we print the username which is the first field $1 and the home path, which is the sixth field $6 from /etc/passwd, and we specify the file that contains that script which is called myscipt with the -f option and surely the separator is specified with capital -F which is the colon.

You can specify multiple commands in the script file, just place each command on a separate line.

This is our file:

$ awk -F: -f testfile /etc/passwd

awk command multiple commands

Here we define a variable that holds a text string used in the print command.

Running Scripts Before Processing Data

If you need to create a header section for the report or something similar, you need to run a script before processing the data.

The BEGIN keyword is used to achieve this. It forces awk to run the script specified after the BEGIN keyword and before awk processes the data:

$ awk 'BEGIN {print "Hello World!"}'

Let’s apply it to something we can see the result:

awk command begin command

Don’t forget the single quotation marks.

Running Scripts After Processing Data

To run a script after processing the data, use the END keyword:

awk command end command

After printing the file contents is finished, the awk command executes the commands in the END section. This is useful, you can use it to add a footer for example.

We can put all these elements together into a nice little script file:

This script uses the BEGIN script to create a header section of the report. It also defines the file separator FS and prints the footer at the end.

$ awk -f myscript  /etc/passwd

awk command complete script

This shows you a small piece of the power available when you use simple awk scripts.

Built-in variables

The awk command uses built-in variables to reference specific features within the program data.

We saw the data field variables $1, $2 $3, etc are used to extract data fields, we also deal with the field separator FS which is by default is a whitespace character, such as space or a tab.

But these are not the only variables, there are more built-in variables.

The following list shows some of the built-in variables:

FIELDWIDTHS     Specifies the list of numbers that defines the exact width (in spaces) of each data field.

RS     Specifies the record separator.

FS     Specifies the field separator.

OFS  Specifies the Output separator.

ORS  Specifies the Output separator.

By default, awk sets the OFS variable to space, By setting the OFS variable, you can use any string to separate data fields in the output:

$ awk 'BEGIN{FS=":"; OFS="-"} {print $1,$6,$7}' /etc/passwd

awk command builtin variables

The FIELDWIDTHS variable enables you to read records without using a field separator character.

In some cases, instead of using a field separator, the data is placed in specific columns within the record. In these cases, you must set the FIELDWIDTHS variable to match the layout of the data in the records.

After you set the FIELDWIDTHS variable, awk ignores the FS and calculates data fields based on the provided field width sizes.

Suppose we have this content:

$ awk 'BEGIN{FIELDWIDTHS="3 4 3"}{print $1,$2,$3}' testfile

awk command field width

Look at the output. The FIELDWIDTHS variable defines four data fields, and awk command parses the data record accordingly. The numbers in each record are separated by the defined field width values.

The RS and ORS variables define how your awk command handles records in the data. By default, awk sets the RS and ORS variables to the newline character which means that each new line of text in the input data stream is a new record.

If your data fields are spread across multiple lines in the data stream like the following:

If you try to read this data using the default FS and RS variable values, awk reads each line as a separate record and interprets each space in the record as a field separator. This is not what you want.

All you need to do is to set the FS variable to the newline character.

Also, you need to set the RS variable to an empty string. The awk command interprets each blank line as a record separator.

$ awk 'BEGIN{FS="\n"; RS=""} {print $1,$3}' addresses

awk command field separator

Awesome! The awk command interpreted each line in the file as a data field and the blank lines as record separators.

Data variables

Besides the built-in variables that you saw, there are some other built-in variables that help you knowing what’s going on with your data and how to extract information from the shell environment:

ARGC     Retrieves the number of command line parameters present.

ARGIND     Retrieves the index in ARGV of the current file being processed.

ARGV     Retrieves an array of command line parameters.

ENVIRON     Retrieves an associative array of the current shell environment variables and their values.

ERRNO     Retrieves the system error if an error occurs when reading or closing input files.

FILENAME    Retrieves the filename of the data file used for input to the awk command.

NF     The total number of data fields in the current record.

NR    Retrieves total count of input records processed.

FNR     Retrieves the current record number in the data file.

IGNORECASE     If set to a non-zero value it will ignore the case of characters.

 

You should know a few of these variables from the previous post about shell scripting.

The ARGC and ARGV variables enable you to get the number of command line parameters.

This can be little tricky because the awk command doesn’t count the script as a part of the command line parameters.

$ awk 'BEGIN{print ARGC,ARGV[1]}' myfile

awk command arguments

The ENVIRON variable uses an associative array to retrieve shell environment variables like this:

awk command data variables

You can use shell variables without ENVIRON variables like this:

$  echo | awk -v home=$HOME '{print "My home is " home}'

awk shell variables

The NF variable enables you to specify the last data field in the record without having to know its position:

$ awk 'BEGIN{FS=":"; OFS=":"} {print $1,$NF}' /etc/passwd

awk command NF

The NF variable contains the numerical value of the last data field in the data file. You can use this variable as a data field variable by placing a dollar sign in front of it.

The FNR and NR variables are similar to each other but somewhat different. The FNR variable holds the number of records processed in the current data file. The NR variable holds the total number of records processed.

Let’s take a look at these two examples to know the difference:

$ awk 'BEGIN{FS=","}{print $1,"FNR="FNR}' myfile myfile

awk command FNR

In this example, the awk command defines two input files. The same file, but processed twice. The output is the first field value and the FNR variable.

Now, let’s add the NR variable and see the difference:

awk command NR FNR

The FNR variable value was reset to 1 when awk processed the second data file, but the NR variable maintained its count in the second data file.

User defined variables

Like any other programming language, awk allows you to define your own variables.

awk user-defined variable name can be any number of letters, digits, and underscores, but it can’t begin with a digit.

You can assign a variable as in shell scripting like this:

awk command user variables

Structured Commands

The awk scripting language supports the standard if-then-else format of the if statement. You must specify a condition for the if statement to evaluate, and enclosed in parentheses.

The testfile contains the following:

10

15

6

33

45

$ awk '{if ($1 > 20) print $1}' testfile

awk command if command

Just that simple.

If you want to execute multiple statements in the if statement, you must enclose them in braces:

awk command multiple statements

The awk if statement also supports the else clause like this:

awk command else

You can use the else clause on a single line, but you need to use a semicolon after the if statement:

awk command else one line

$ awk '{if ($1 > 20) print $1 * 2; else print $1 / 2}' testfile

While loop

The while loop enables you to iterate over a set of data, checking a condition that stops the iteration.

cat myfile

124 127 130

112 142 135

175 158 245

awk command while loop

The while statement iterates over the data fields in the record and adds each value to the total variable and increments the counter variable i by 1.

When the counter value becomes 4, the while condition becomes FALSE, and the loop terminates, going to the next statement in the script. That statement estimates the average and prints it.

The awk scripting language supports using the break and continue statements in while loops, allowing you to jump out of the middle of the loop.

awk command break

The for loop

The for loop is a common technique used in many programming languages for looping.

The awk scripting language supports the for loops:

awk command for loop

Formatted Printing

The printf command in awk allows you to print formatted output.

It defines how the formatted output should appear, using both text elements and format specifiers.

A format specifier is a special code that implies what type of variable is displayed and how to display it. The awk command uses each format specifier as a placeholder for each variable listed in the command.

The format specifiers are written like this:

%[modifier]control-letter

This list shows the format specifiers you can use with printf:

c              Prints numeric output as an ASCII character.

d             Prints an integer value.

i              Prints an integer value (same as d).

e             Prints numeric output in scientific notation.

f               Prints a floating-point value.

g             Prints either scientific notation or floating point.

o             Prints an octal value.

s             Prints a text string.

Here we use printf to format our output:

awk command printf

Here as an example, we display a large value using scientific notation %e.

We are not going to try every format specifier. You know the concept.

Built-In Functions

The awk scripting language provides a few built-in functions that perform mathematical, string, and time functions. You can utilize these functions in your awk scripts.

Mathematical functions

If you love math, these are some of the mathematical functions you can use with awk:

cos(x) | exp(x) | int(x) | log(x) | rand() | sin(x) | sqrt(x)

And they can be used normally:

$ awk 'BEGIN{x=exp(5); print x}'

awk command math functions

String functions

There are many string functions, you can check the list, but we will examine one of them as an example and the rest is the same:

$ awk 'BEGIN{x = "likegeeks"; print toupper(x)}'

awk command string functions

The function toupper converts character case to upper case for the passed string.

User Defined Functions

You can create your own functions for using in awk scripts, just define them and use them.

awk command user defined functions

Here we define a function called myprint, then we use it in our script to print output using printf function.

I hope you like the post.

Thank you.

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  • clfapujc

    I can’t get the output shown on the first terminal screen shot. Is the command correct?

    edit. I get it now. Sorry for disturbing in the comments 😀

    • sigzero

      You have to hit ENTER. Awk won’t print that per the instructions.

    • No problem you are welcome all time.
      Regards

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  • Amani Hamis

    I’d like to suggest one correction, if I may. I got a little confused with the NF built-in variable, which made me read the man page for awk, and I think the NF built-in holds the “total number of fields in the ‘current input record’ ” and not ‘data file’. That made me confused a little.
    Thanks for the work!

    • Thank you very much Amani for your notice, appreciate it.