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LPI Linux Essentials Exam 010-160 - Topic 3.2 - cut Command

The cut command in Linux is a versatile utility for text processing. It serves to remove or "cut out" particular sections from each line of a file or an input stream, which can then be displayed on the standard output. This utility can be incredibly useful when you're dealing with delimited data files, extracting columns from a table, or parsing logs.

Syntax

The basic syntax of the cut command is as follows:

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

If no file is specified, cut reads from the standard input.

Key Options

  • -d, --delimiter=DELIM: Use DELIM instead of the TAB character for field delimiter.
  • -f, --fields=LIST: Select only these fields.
  • -c, --characters=LIST: Select only these characters.
  • --complement: Complement (invert) the set of selected bytes, characters, or fields.

Using cut with /etc/passwd

The /etc/passwd file is a text-based database that describes user account information. Each line in this file represents a single user account and contains various fields separated by colons :. Below is a generic entry for illustration:

user1:x:1001:1001:User One:/home/user1:/bin/shell

Extract Usernames

In this exercise, you will use the cut command to extract all usernames from the /etc/passwd file.

Exercise: Extract usernames:

cut -d: -f1 /etc/passwd
  • -d: specifies that the delimiter is a colon.
  • -f1 tells cut to display only the first field, which is the username.

Extract User Shells

To obtain a list of all the shells used by users, follow the below exercise:

Exercise: List shells in /etc/passwd:

cut -d: -f7 /etc/passwd
  • -f7 denotes that we are interested in the 7th field, which is the shell assigned to the user.

Extract Multiple Fields

Suppose you want both the username and the shell fields. You can specify multiple fields like so:

Exercise: Display username and their shell:

cut -d: -f1,7 /etc/passwd
  • f1,7 means that cut will display the 1st and the 7th fields.

Complementing the Selection

The --complement option allows you to invert the selection. For example, if you wish to display all fields except the 2nd one (which typically contains an 'x' for the password), you can follow:

Exercise: Display all fields except historic password field:

cut -d: -f2 --complement /etc/passwd

Select by Character Position

If you want to select specific characters rather than fields, you can do so using the -c option.

Exercise: Display the first 10 characters from each line of /etc/passwd:

cut -c1-10 /etc/passwd
  • Here, -c1-10 means you want characters from position 1 to 10.

Summary

The cut command is an essential tool for text processing in Linux. By understanding how to use its various options, you can efficiently parse and manipulate text data, as demonstrated with exercises on the /etc/passwd file.

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