1. Overview
As Linux users, we frequently perform various operations on files. For example, one of the common operations is to display specific columns from a file.
In this tutorial, we’ll discuss the various ways to achieve this.
2. Display Single Column
Let’s create a file to use as an example. The input.txt file contains the output of the ls command in the long listing format:
$ cat input.txt
-rw-r--r-- 1 jarvis jarvis 200M Apr 27 22:04 file1.dat
-rw-r--r-- 1 jarvis jarvis 400M Apr 27 22:04 file2.dat
-rw-r--r-- 1 jarvis jarvis 500M Apr 27 22:04 file3.dat
-rw-r--r-- 1 jarvis jarvis 600M Apr 27 22:04 file4.dat
-rw-r--r-- 1 jarvis jarvis 700M Apr 27 22:04 file5.dat
We can use the awk command to display specific columns. Let’s print the 5th column from the file:
$ awk '{print $5}' input.txt
200M
400M
500M
600M
700M
Let’s see the option we used in the awk command:
- print: it’s awk’s built-in function which prints text to the standard output stream
- $5: it represents files size from the 5th column
Note that awk uses $N to represent the Nth column. For example, $2 represents the 2nd column.
We can also use the cut command to display specific columns. Let’s print the same column using the cut command:
$ cut -d' ' -f5 input.txt
200M
400M
500M
600M
700M
Let’s take a look at the option we used in the cut command:
- -d: it represents the field delimiter. Its default value is a tab character
- –f5: it represents the file size from the 5th column
3. Display Multiple Columns
We can use awk to display multiple columns as well. Let’s print the file name and its size:
$ awk '{print $9 " " $5}' input.txt
file1.dat 200M
file2.dat 400M
file3.dat 500M
file4.dat 600M
file5.dat 700M
Let’s see the options we used in the awk command:
- $9: it represents file name from the 9th column
It’s also possible to use the cut command to display multiple columns. For example, we can specify multiple columns using a comma separate list as follows:
$ cut -d' ' -f9,5 input.txt
200M file1.dat
400M file2.dat
500M file3.dat
600M file4.dat
700M file5.dat
Note that it’s not possible to rearrange column order with the cut command. Selected input is written in the same order that it is read.
4. Display Range of Columns
Sometimes it’s convenient to use a loop when columns to be displayed are in large numbers. Let’s print all columns within the range of 3 to 8:
$ awk '{ for (i = 3; i <= 8; ++i) printf $i" "; print ""}' input.txt
jarvis jarvis 200M Apr 27 22:04
jarvis jarvis 400M Apr 27 22:04
jarvis jarvis 500M Apr 27 22:04
jarvis jarvis 600M Apr 27 22:04
jarvis jarvis 700M Apr 27 22:04
Let’s take a look at the options we used in the awk command:
- for: it’s a looping construct of the awk
- printf: it’s awk’s built-in function which prints formatted text to the standard output stream
We can use the cut command to achieve the same result. It allows us to specify a range of columns using hyphen character as follows:
$ cut -d' ' -f3-8 input.txt
jarvis jarvis 200M Apr 27 22:04
jarvis jarvis 400M Apr 27 22:04
jarvis jarvis 500M Apr 27 22:04
jarvis jarvis 600M Apr 27 22:04
jarvis jarvis 700M Apr 27 22:04
5. Changing the awk Field Separator
By default, awk uses a space character as a column separator. However, we can modify it according to our requirements. First, let’s modify our original input file by replacing spaces with commas. Now modified file looks like this:
$ cat input.txt
-rw-r--r--,1,jarvis,jarvis,200M,Apr 27 22:04,file1.dat
-rw-r--r--,1,jarvis,jarvis,400M,Apr 27 22:04,file2.dat
-rw-r--r--,1,jarvis,jarvis,500M,Apr 27 22:04,file3.dat
-rw-r--r--,1,jarvis,jarvis,600M,Apr 27 22:04,file4.dat
-rw-r--r--,1,jarvis,jarvis,700M,Apr 27 22:04,file5.dat
Let’s print the file name, its size, and timestamp using a comma as a column separator:
$ awk -F"," '{print $7 " " $5 " " $6}' input.txt
file1.dat 200M Apr 27 22:04
file2.dat 400M Apr 27 22:04
file3.dat 500M Apr 27 22:04
file4.dat 600M Apr 27 22:04
file5.dat 700M Apr 27 22:04
Let’s see the option we used in the awk command:
- -F: it represents the field separator
Note that in the 6th column, spaces are not replaced with the commas intentionally to demonstrate the use of field separator.
6. Conclusion
In this tutorial, we discussed various examples to display specific columns from a file. The commands showed in this tutorial can be used in day-to-day life while working with the Linux system.