Mastering the Linux “cut” Command: Byte, Character, and Field Extraction Techniques

Command Overview

The cut command trims and extracts portions of each line from a file or standard input based on specified criteria. It is often combined with other commands to pull out exactly the data you need.

Basic Syntax

cut [options] file

Key options include: -b: split by bytes -c: split by characters -d: define a custom delimiter (default is a tab) -f: select fields (used with -d) -n: suppress splitting of multibyte characters (used with -b)

Byte‑Based Extraction

Given a file test1.txt containing the days of the week, the following examples demonstrate byte selection:

1. First byte of each line

[alvin@VM_0_16_centos cut]$ cut -b 1 test1.txt
M
T
W
T
F
S
S

2. Bytes 2, 4, 6

[alvin@VM_0_16_centos cut]$ cut -b 2,4,6 test1.txt
ody
usa
ens
hrd
rdy
aud
udy

3. Byte range 3‑6

[alvin@VM_0_16_centos cut]$ cut -b 3-6 test1.txt
nday
esda
dnes
ursd
iday
turd
nday

4. Bytes 1‑3 and 6

[alvin@VM_0_16_centos cut]$ cut -b 1-3,6 test1.txt
Mony
Tuea
Weds
Thud
Friy
Satd
Suny

5. First three bytes or everything after the third byte

[alvin@VM_0_16_centos cut]$ cut -b -3 test1.txt
Mon
Tue
Wed
Thu
Fri
Sat
Sun
[alvin@VM_0_16_centos cut]$ cut -b 3- test1.txt
nday
esday
dnesday
ursday
iday
turday
nday

Character‑Based Extraction

Bytes and characters differ for multibyte text. For example, the Chinese character 啊 occupies multiple bytes but counts as a single character. Use -c for characters or -nb to treat multibyte characters as single units.

Given test2.txt with Chinese lines, extracting the second character works as follows:

[alvin@VM_0_16_centos cut]$ cut -nb 2 test2.txt
许
望
灰
哥
[alvin@VM_0_16_centos cut]$ cut -c 2 test2.txt
许
望
灰
哥

Field‑Based Extraction

Fields are sections separated by a delimiter. Using /etc/passwd as an example, each line’s fields are delimited by colons ( :). To extract the first field (the username) from the first five lines:

[alvin@VM_0_16_centos cut]$ cat /etc/passwd | head -n 5 | cut -d : -f 1
root
bin
daemon
adm
lp

The delimiter can be changed with -d to commas, spaces, or any other character, but note that cut cannot handle multiple consecutive delimiters reliably, which is a limitation of the command.