Awk tool:
Awk is one of the most powerful tools in Unix used for processing the rows and columns in a file. Awk has built in string functions and associative arrays. Awk supports most of the operators, conditional blocks, and loops available in C language.
One of the good things is that you can convert Awk scripts into Perl scripts using a2p utility.
The basic syntax of AWK:
awk 'BEGIN {start_action} {action} END {stop_action}' filename
cat /etc/passwd | awk -F : '{ if($3 >= '500') print $1;}' | more
Above command will only show normal users, not systems user
Here the actions in the begin block are performed before processing the file and the actions in the end block are performed after processing the file. The rest of the actions are performed while processing the file.
Examples:
Create a file input_file with the following data. This file can be easily created using the output of ls -l.
From the data, you can observe that this file has rows and columns. The rows are separated by a new line character and the columns are separated by a space characters. We will use this file as the input for the examples discussed here.
1. awk '{print $1}' input_file
Here $1 has a meaning. $1, $2, $3... represents the first, second, third columns... in a row respectively. This awk command will print the first column in each row as shown below.
To print the 4th and 6th columns in a file use awk '{print $4,$5}' input_file
Here the Begin and End blocks are not used in awk. So, the print command will be executed for each row it reads from the file. In the next example we will see how to use the Begin and End blocks.
2. awk 'BEGIN {sum=0} {sum=sum+$5} END {print sum}' input_file
This will prints the sum of the value in the 5th column. In the Begin block the variable sum is assigned with value 0. In the next block the value of 5th column is added to the sum variable. This addition of the 5th column to the sum variable repeats for every row it processed. When all the rows are processed the sum variable will hold the sum of the values in the 5th column. This value is printed in the End block.
3. In this example we will see how to execute the awk script written in a file. Create a file sum_column and paste the below script in that file
Now execute the the script using awk command as
awk -f sum_column input_file.
This will run the script in sum_column file and displays the sum of the 5th column in the input_file.
4. awk '{ if($9 == "t4") print $0;}' input_file
This awk command checks for the string "t4" in the 9th column and if it finds a match then it will print the entire line. The output of this awk command is
5. awk 'BEGIN { for(i=1;i<=5;i++) print "square of", i, "is",i*i; }'
This will print the squares of first numbers from 1 to 5. The output of the command is
Notice that the syntax of “if” and “for” are similar to the C language.
Awk Built in Variables:
You have already seen $0, $1, $2... which prints the entire line, first column, second column... respectively. Now we will see other built in variables with examples.
FS - Input field separator variable:
So far, we have seen the fields separted by a space character. By default Awk assumes that fields in a file are separted by space characters. If the fields in the file are separted by any other character, we can use the FS variable to tell about the delimiter.
6. awk 'BEGIN {FS=":"} {print $2}' input_file
OR
awk -F: '{print $2} input_file
This will print the result as
OFS - Output field separator variable:
By default whenever we printed the fields using the print statement the fields are displayed with space character as delimiter. For example
7. awk '{print $4,$5}' input_file
The output of this command will be
We can change this default behavior using the OFS variable as
awk 'BEGIN {OFS=":"} {print $4,$5}' input_file
Note: print $4,$5 and print $4$5 will not work the same way. The first one displays the output with space as delimiter. The second one displays the output without any delimiter.
NF - Number of fileds variable:
The NF can be used to know the number of fields in line
8. awk '{print NF}' input_file
This will display the number of columns in each row.
NR - number of records variable:
The NR can be used to know the line number or count of lines in a file.
9. awk '{print NR}' input_file
This will display the line numbers from 1.
10. awk 'END {print NR}' input_file
This will display the total number of lines in the file.
Awk is one of the most powerful tools in Unix used for processing the rows and columns in a file. Awk has built in string functions and associative arrays. Awk supports most of the operators, conditional blocks, and loops available in C language.
One of the good things is that you can convert Awk scripts into Perl scripts using a2p utility.
The basic syntax of AWK:
awk 'BEGIN {start_action} {action} END {stop_action}' filename
cat /etc/passwd | awk -F : '{ if($3 >= '500') print $1;}' | more
Above command will only show normal users, not systems user
Here the actions in the begin block are performed before processing the file and the actions in the end block are performed after processing the file. The rest of the actions are performed while processing the file.
Examples:
Create a file input_file with the following data. This file can be easily created using the output of ls -l.
-rw-r--r-- 1 center center 0 Dec 8 21:39 p1 -rw-r--r-- 1 center center 17 Dec 8 21:15 t1 -rw-r--r-- 1 center center 26 Dec 8 21:38 t2 -rw-r--r-- 1 center center 25 Dec 8 21:38 t3 -rw-r--r-- 1 center center 43 Dec 8 21:39 t4 -rw-r--r-- 1 center center 48 Dec 8 21:39 t5
From the data, you can observe that this file has rows and columns. The rows are separated by a new line character and the columns are separated by a space characters. We will use this file as the input for the examples discussed here.
1. awk '{print $1}' input_file
Here $1 has a meaning. $1, $2, $3... represents the first, second, third columns... in a row respectively. This awk command will print the first column in each row as shown below.
-rw-r--r-- -rw-r--r-- -rw-r--r-- -rw-r--r-- -rw-r--r-- -rw-r--r--
To print the 4th and 6th columns in a file use awk '{print $4,$5}' input_file
Here the Begin and End blocks are not used in awk. So, the print command will be executed for each row it reads from the file. In the next example we will see how to use the Begin and End blocks.
2. awk 'BEGIN {sum=0} {sum=sum+$5} END {print sum}' input_file
This will prints the sum of the value in the 5th column. In the Begin block the variable sum is assigned with value 0. In the next block the value of 5th column is added to the sum variable. This addition of the 5th column to the sum variable repeats for every row it processed. When all the rows are processed the sum variable will hold the sum of the values in the 5th column. This value is printed in the End block.
3. In this example we will see how to execute the awk script written in a file. Create a file sum_column and paste the below script in that file
#!/usr/bin/awk -f
BEGIN {sum=0}
{sum=sum+$5}
END {print sum}
Now execute the the script using awk command as
awk -f sum_column input_file.
This will run the script in sum_column file and displays the sum of the 5th column in the input_file.
4. awk '{ if($9 == "t4") print $0;}' input_file
This awk command checks for the string "t4" in the 9th column and if it finds a match then it will print the entire line. The output of this awk command is
-rw-r--r-- 1 pcenter pcenter 43 Dec 8 21:39 t4
5. awk 'BEGIN { for(i=1;i<=5;i++) print "square of", i, "is",i*i; }'
This will print the squares of first numbers from 1 to 5. The output of the command is
square of 1 is 1 square of 2 is 4 square of 3 is 9 square of 4 is 16 square of 5 is 25
Notice that the syntax of “if” and “for” are similar to the C language.
Awk Built in Variables:
You have already seen $0, $1, $2... which prints the entire line, first column, second column... respectively. Now we will see other built in variables with examples.
FS - Input field separator variable:
So far, we have seen the fields separted by a space character. By default Awk assumes that fields in a file are separted by space characters. If the fields in the file are separted by any other character, we can use the FS variable to tell about the delimiter.
6. awk 'BEGIN {FS=":"} {print $2}' input_file
OR
awk -F: '{print $2} input_file
This will print the result as
39 p1 15 t1 38 t2 38 t3 39 t4 39 t5
OFS - Output field separator variable:
By default whenever we printed the fields using the print statement the fields are displayed with space character as delimiter. For example
7. awk '{print $4,$5}' input_file
The output of this command will be
center 0 center 17 center 26 center 25 center 43 center 48
We can change this default behavior using the OFS variable as
awk 'BEGIN {OFS=":"} {print $4,$5}' input_file
center:0 center:17 center:26 center:25 center:43 center:48
Note: print $4,$5 and print $4$5 will not work the same way. The first one displays the output with space as delimiter. The second one displays the output without any delimiter.
NF - Number of fileds variable:
The NF can be used to know the number of fields in line
8. awk '{print NF}' input_file
This will display the number of columns in each row.
NR - number of records variable:
The NR can be used to know the line number or count of lines in a file.
9. awk '{print NR}' input_file
This will display the line numbers from 1.
10. awk 'END {print NR}' input_file
This will display the total number of lines in the file.
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