Linux cut command explained

The cut command in Linux and Unix is ​​used to cut out some parts from each line of a file and output them to the standard output. We can use the cut command to extract a portion of the content from a line of string in units of bytes, characters, fields (separators), etc.

In this article, we will learn about the use of cut command through some examples. These usage methods are also very commonly used in our daily work.

Cut Command and Syntax

The basic syntax of the cut command is as follows:

$ cut OPTION... [FILE]...

Let's first take a look at some options of cut. The cut command must be specified with options to be executed.

-f : Extract the specified field. The cut command uses Tab as the default separator.

-d : Tab is the default delimiter, use this option to specify your own delimiter.

-b : Extract the specified bytes, or specify a range.

-c : Extract the specified characters, which can be a list of numbers separated by commas or a range of numbers separated by hyphens.

–complement : complement the selected part, i.e. invert the selection.

–output-delimiter : Change the delimiter used when outputting.

--only-delimited : Do not output columns that do not contain delimiters.

We take the following text file named context.txt and /etc/passwd file as examples to illustrate.

$ cat content.txt 
Ubuntu Linux
Microsoft Windows
OsX El Capitan
Unix
FreeBSD

How to specify the delimiter

The most commonly used options are the combination of -d and -f , which extracts content based on the delimiter specified by -d and the fields listed by -f .

For example, in this example, only the first field of each line of the /etc/passwd file is printed, and the separator used is :

$ cut -d':' -f1 /etc/passwd
root
bin
daemon
adm
lp
sync
shutdown
halt
mail
operator
games
alvin
liangxu
...

In the following example we print the first field of the content.txt file using spaces as delimiters.

$ cut -d " " -f 1 content.txt 
Ubuntu
Microsoft
OxY
Unix
FreeBSD

In the example below we extract multiple fields. Here, we extract the first and sixth fields from the line containing the string /bin/bash in the file /etc/passwd using the colon (:) delimiter.

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1,6
root:/root
alvin:/home/alvin

To display a range of fields, you can specify the starting and ending fields, separated by a hyphen (-), as shown below:

$ grep "/bin/bash" /etc/passwd | cut -d':' -f1-4,6,7
root:x:0:0:/root:/bin/bash
alvin:x:1000:1000:/home/alvin:/bin/bash

How to complete the selected output

To complete the selected output fields (i.e., deselect them), use the --complement option. This option outputs all fields except the specified fields.

The following example prints all fields except the second field in the line containing /bin/bash in the /etc/passwd file:

$ grep "/bin/bash" /etc/passwd | cut -d':' --complement -f2
root:0:0:root:/root:/bin/bash

How to specify the output delimiter

Use --output-delimiter to specify the output delimiter. The input delimiter is specified by -d , and the output delimiter is the same as the input delimiter by default.

Let's first use the following example to test the output when the output delimiter is not specified;

$ cut -d: -f1,7 /etc/passwd | sort | uniq -u
_apt:/usr/sbin/nologin
backup:/usr/sbin/nologin
bin:/usr/sbin/nologin
daemon:/usr/sbin/nologin
dnsmasq:/usr/sbin/nologin
games:/usr/sbin/nologin
gnats:/usr/sbin/nologin
irc:/usr/sbin/nologin
landscape:/usr/sbin/nologin
list:/usr/sbin/nologin
lp:/usr/sbin/nologin
lxd:/bin/false

Now we add the --output-delimiter option to specify the output delimiter as a space:

$ cut -d: -f1,7 --output-delimiter ' ' /etc/passwd | sort | uniq -u
_apt /usr/sbin/nologin
backup /usr/sbin/nologin
bin /usr/sbin/nologin
daemon /usr/sbin/nologin
dnsmasq /usr/sbin/nologin
games /usr/sbin/nologin
gnats /usr/sbin/nologin
irc /usr/sbin/nologin
landscape /usr/sbin/nologin
list /usr/sbin/nologin
lp /usr/sbin/nologin
lxd /bin/false

Let's test another example and use a separator to print one field per line.

We specify --output-delimiter as $'\n' to indicate a newline.

The output is:

$ grep root /etc/passwd | cut -d':' -f1,6,7 --output-delimiter=$'\n'
root
/root
/bin/bash
operator
/root
/sbin/nologin

How to extract content as characters

The -c option can be used to extract based on character position. Note that spaces and Tab are also treated as characters.

Print the first character of each line of the context.txt file, as follows:

$ cut -c 1 content.txt
U
M
O
U
F

The following shows the first seven characters of each line of the context.txt file;

$ cut -c 1-7 content.txt
Ubuntu
Microso
XOt
Unix
FreeBSD

Let's test again by specifying only the start or end position.

The following extracts the second to last characters:

$ cut -c2- content.txt
Ubuntu Linux
Microsoft Windows
sX El Capitan
nix
reeBSD

Extract the first to fourth characters:

cut -c-4 content.txt
Ubun
Micr
OxY
Unix
Free

How to extract by bytes

Use the -b option to select part of a line by specifying byte positions, using commas to separate each specified position, or - to specify a range.

The following example extracts the first, second, and third bytes of each line in the content.txt file:

$ cut -b 1,2,3 content.txt 
Ubu
Mic
OxY
Uni
Fre

We can also list a range using the following command;

$ cut -b 1-3,5-7 content.txt 
Ubutu 
Micoso
XOtM 
Uni
FreBSD

Some practical examples

cut is a useful command that is often used in conjunction with other Linux or Unix commands.

For example, if you want to extract USER, PID and COMMAND from the ps command:

ps -L un | tr -s " " | cut -d " " -f 2,3,14-
USER PID COMMAND
0 676 /sbin/agetty -o -p --\u --keep-baud 115200,38400,9600 ttyS0 vt220
0 681 /sbin/agetty -o -p --\u --noclear tty1 linux
0 23174 -bash
0 26737 ps -L un
0 26738 tr -s
0 26739 cut -d -f 2,3,14-

Let's test another example to extract the total, used, and free values ​​of the memory and save them to a file.

$ free -m | tr -s ' ' | sed '/^Mem/!d' | cut -d" " -f2-4 >> memory.txt
$ cat memory.txt
985 86 234

Summarize

The cut command can be connected to many other Linux or Unix commands through pipes. One or more filters can be piped through for additional text processing.

One of the limitations of the cut command is that it does not support specifying multiple characters as delimiters. Multiple spaces are counted as multiple field separators, so the tr command must be used before the cut command to obtain the desired output.

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