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 something powerful, giving you more control to process data. This is where awk command comes in.

The awk command or GNU awk in specific provides a scripting language for text processing. With awk scripting language, you can make the following:

  • Define variables.
  • Use string and arithmetic operators.
  • Use control flow and loops.
  • Generate formatted reports.

Actually, you can process log files that contain maybe millions of lines to output a readable report that you can benefit from.

 

 

Awk Options

The awk command is used like this:

$ awk options program file

Awk can take the following options:

-F fs     To specify a file separator.

-f file     To specify a file that contains awk script.

-v var=value     To declare a variable.

We will see how to process files and print results using awk.

 

Read AWK Scripts

To define an awk script, use braces surrounded by single quotation marks like this:

$ 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, press The Ctrl+D. Looks tricky, don’t panic, the best is yet to come.

 

Using Variables

With awk, you can process text files. Awk assigns some variables for each data field found:

  • $0 for the whole line.
  • $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

The above example prints the first word of each line.

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 prints the first field in the passwd file. We use the colon as a separator because the passwd file uses it.

 

Using Multiple Commands

To run multiple commands, separate them with a semicolon like this:

$ echo "Hello Tom" | awk '{$2="Adam"; print $0}'

awk multiple commands

The first command makes the $2 field equals Adam. The second command prints the entire line.

 

Reading The Script From a File

You can type your awk script in a file and specify that file using 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 and his home path from /etc/passwd, and surely the separator is specified with capital -F which is the colon.

You can your awk script file like this:

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

awk command multiple commands

Awk Preprocessing

If you need to create a title or a header for your result or so. You can use the BEGIN keyword to achieve this. It runs before processing the data:

$ awk 'BEGIN {print "Report Title"}'

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

awk command begin command

 

Awk Postprocessing

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

awk command end command

This is useful, you can use it to add a footer for example.

Let’s combine them together in a script file:

First, the top section is created using BEGIN keyword. Then we define the FS and print the footer at the end.

$ awk -f myscript  /etc/passwd

awk command complete script

 

Built-in Variables

We saw the data field variables $1, $2 $3, etc are used to extract data fields, we also deal with the field separator FS.

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 field width.

RS     Specifies the record separator.

FS     Specifies the field separator.

OFS  Specifies the Output separator.

ORS  Specifies the Output separator.

By default, the OFS variable is the space, you can set the OFS variable to specify the separator you need:

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

awk command builtin variables

Sometimes, the fields are distributed without a fixed separator. In these cases, FIELDWIDTHS variable solves the problem.

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 output fields are 3 per line and each field length is based on what we assigned by FIELDWIDTH exactly.

Suppose that your data are distributed on different lines like the following:

In the above example, awk fails to process fields properly because the fields are separated by new lines and not spaces.

You need to set the FS to the newline (\n) and the RS to a blank text, so empty lines will be considered separators.

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

awk command field separator

Awesome! we can read the records and fields properly.

 

More Variables

There are some other variables that help you to get more information:

ARGC     Retrieves the number of passed parameters.

ARGV     Retrieves the command line parameters.

ENVIRON     Array of the shell environment variables and corresponding values.

FILENAME    The file name that is processed by awk.

NF     Fields count of the line being processed.

NR    Retrieves total count of processed records.

FNR     The record which is processed.

IGNORECASE     To ignore the character case.

You can review the previous post shell scripting to know more about these variables.

Let’s test them.

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

awk command arguments

The ENVIRON variable retrieves the shell environment variables like this:

awk command data variables

You can use bash variables without ENVIRON variables like this:

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

awk shell variables

The NF variable specifies the last field in the record without knowing its position:

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

awk command NF

The NF variable can be used as a data field variable if you type it like this: $NF.

Let’s take a look at these two examples to know the difference between FNR and NR variables:

$ 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, check the NR variable and see the difference:

awk command NR FNR

The FNR variable becomes 1 when comes to the second file, but the NR variable keeps its value.

 

User Defined Variables

Variable names could be anything, but it can’t begin with a number.

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

awk command user variables

 

Structured Commands

The awk scripting language supports if conditional statement.

The testfile contains the following:

10

15

6

33

45

$ awk '{if ($1 > 30) print $1}' testfile

awk command if command

Just that simple.

You should use braces if you want to run multiple statements:

awk command multiple statements

You can use else statements like this:

awk command else

Or type them on the same line and separate the if statement with a semicolon like this:

awk command else one line

While Loop

You can use the while loop to iterate over data with a condition.

cat myfile

124 127 130

112 142 135

175 158 245

118 231 147

awk command while loop

The while loop runs and every time it adds 1 to the sum variable until the i variables becomes 4.

You can exit the loop using break command like this:

awk command break

The for Loop

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 using format specifiers.

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 a string.

d             Prints an integer value.

e             Prints scientific numbers.

f               Prints float values.

o             Prints an octal value.

s             Prints a text string.

Here we use printf to format our output:

awk command printf

Here is an example of printing scientific numbers.

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

 

Built-In Functions

Awk provides several built-in functions like:

Mathematical Functions

If you love math, you can use these functions in your awk scripts:

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

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 define your function and use them like this:

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.

  • Pingback: 30 Examples For Awk Command In Text Processing | Ace Infoway()

  • 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

  • Pingback: 35+ Examples of Regex Patterns Using sed and awk in Linux - Path to Geek()

  • 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.

  • jrduboc

    Can I also suggest a correction ?

    >>>>>>>>
    $ echo “Hello Tom” | awk ‘{$4=”Adam”; print $0}’

    awk multiple commands
    The first command makes the $2 field equals Adam. The second command prints the entire line.
    >>>>>>>>

    The command to make $2 field equal to Adam should be awk ‘{$2=”Adam”; print $0}’