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 programming language instead of just editor commands. Within the awk programming language, you can do the following:

  • Define variables to store data.
  • Use arithmetic and string operators to operate on data.
  • Use structured programming concepts and control flow, such as if-then statements and loops, to add logic to your text processing.
  • 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     Specifies a file separator for the fields in a line.

-f file     specifies a file name to read the program from.

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

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

mr N     Specifies the maximum record size in the data file.

-W     keyword Specifies the compatibility mode or warning level for awk.

The real power of awk is in the program script. You can write scripts to read the data within a text line and then manipulate and display the data to create any type of output report.

Reading the program script from the command line

Awk program script is defined by opening and closing braces. You must place script commands between the two braces and because the awk command line assumes that the script is a single text string, you must enclose your script in single quotation marks like this:

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

If you run this command, nothing will happen!! And this because no filename was defined in the command line.

The awk command retrieves data from STDIN. When you run the program, it just waits for the text to comes in via STDIN.

If you type a line of text and press Enter, the awk command runs the program through the text. Just like the sed editor, the awk command executes the program script on each line of text available in the data stream. Because the program script is set to display a fixed text string, you get the same text output.

$ 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 generates an EOF character. Maybe you disappointed with this example but wait for the awesomeness.

Using data field variables

One of the main features of awk is its ability to manipulate data in a text file. It does this by automatically assigning a variable to each element in a line. By default, awk assigns the following variables to each data field it detects in a line of text:

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

Each data field is defined in a text line by a field separation character.

The default field separation character in awk is any whitespace character like tab or space.

Look at the following file and see how awk deals with 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. Because the /etc/passwd file uses a colon to separate the data fields, I use the colon as a separator.

Using multiple commands

It wouldn’t be very useful if you execute only one command.

The awk programming language allows you to combine commands in a normal program.

To run multiple commands on the command line, just place a semicolon between each command:

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

awk multiple commands

The first command assigns a value to the $4 field variable. The second command prints the entire line.

Reading the program from a file

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

Our file contains this script:

{print $1 " has a  home directory 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. You don’t need to use semicolons.

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

Sometimes, you may need to run a script before processing data, such as creating a header section for the report or something similar.

The BEGIN keyword is used to accomplish this. It forces awk to execute the script specified after the BEGIN keyword and before awk reads the data:

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

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

awk command begin command

Now, after awk command executes the BEGIN script, it uses the second script to process any file data. Be careful when doing this; both of the scripts are still considered one text string on the awk command line. You need to put your single quotation marks accordingly.

Running scripts after processing data

The END keyword allows you to specify a program script that awk command executes after reading the data:

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     A space-separated list of numbers defining the exact width (in spaces) of each data field.

FS     Input field separator character.

RS     Input record separator character.

OFS  Output field separator character.

ORS  Output record separator character.

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 allows you to read records without using a field separator character.

In some situations, instead of using a field separator, the data is placed in specific columns within the record. In these instances, 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 5 2 5"}{print $1,$2,$3,$4}'

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.

Sometimes, you run into situations where 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.

To solve this problem, you need to set the FS variable to the newline character. This indicates that each line in the data is a separate field.

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     The number of command line parameters present.

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

ARGV     An array of command line parameters.

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

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

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

FNR     The current record number in the data file.

IGNORECASE     If set to a non-zero value ignores the case of characters in strings used in the awk command.

NF     The total number of data fields in the data file.

NR     The number of input records processed.

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

The ARGC and ARGV variables allow you to retrieve 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. It defines the same input file twice. The script prints the first data field value and the current value of 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 programming 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 must 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 allows 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 is equal to 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 programming 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 programming language supports the for loops:

awk command for loop

By defining the iteration counter in the for loop, you don’t have to worry about incrementing it yourself as you did when using the while statement.

Formatted Printing

The printf command in awk allows you to specify detailed instructions on how to display data.

It specifies exactly 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 first format specifier matches the first variable listed; the second one matches the second variable, and so on.

The format specifiers use the following format:

%[modifier]control-letter

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

c              Displays a number as an ASCII character

d             Displays an integer value

i               Displays an integer value (same as d)

e             Displays a number in scientific notation

f              Displays a floating-point value

g              Displays either scientific notation or floating point, whichever is shorter

o             Displays an octal value

s              Displays 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 programming 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)    The cosine of x, with x specified in radians.

exp(x)   The exponential of x.

int(x)     The integer part of x, truncated toward 0.

log(x)    The natural logarithm of x.

rand()   A random floating point value larger than 0 and less than 1.

sin(x)     The sine of x, with x specified in radians.

sqrt(x)  The square root of 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.