The 'cut' Command in Unix/Linux

The cut command is a Unix utility used for extracting specific sections (columns) of lines from files or standard input. It is particularly useful when dealing with structured text data that is delimited by a common character, such as tabs or spaces. cut allows you to define the delimiter and specify the fields (columns) to extract. Here's a detailed explanation of the cut command with examples:

Basic Syntax:

cut [options] [file]

• options: Optional flags to control the behavior of cut.
• [file]: The input file to process. If not provided, cut reads from standard input.

Common Options:

1. -f or --fields: Specifies the field(s) to extract. Fields are separated by the delimiter.

   • Example: -f 1,3 extracts the first and third fields.

2. -d or --delimiter: Specifies the delimiter character that separates fields. The default delimiter is the tab character (\t).

   • Example: -d , specifies a comma as the delimiter.

3. -c or --characters: Specifies the character(s) or byte range to extract.

   • Example: -c 1-5 extracts characters 1 through 5.

4. -s or --only-delimited: Suppresses lines that do not contain the delimiter character.

Examples of cut Usage:

1. Extracting Fields from a CSV File:

   • Extract the first and third fields from a CSV file.

   cut -d ',' -f 1,3 data.csv
   

2. Extracting Characters from a Text File:

   • Extract characters 1 to 5 from each line of a file.

   cut -c 1-5 file.txt
   

3. Extracting Fields from a TSV (Tab-Separated Values) File:

   • Extract the second field from a TSV file.

   cut -f 2 data.tsv
   

4. Extracting Fields with a Different Delimiter:

   • Extract the second field from a colon-separated file.

   cut -d ':' -f 2 file.txt
   

5. Suppressing Non-Delimited Lines:

   • Extract the second field from lines that contain a delimiter.

   cut -d ':' -f 2 -s file.txt
   

6. Extracting Columns from ps Output:

   • Extract the process ID (PID) and command columns from the ps command output.

   ps aux | cut -d ' ' -f 1,11
   

7. Extracting Users from /etc/passwd:

   • Extract the usernames from the /etc/passwd file (assuming the usernames are in the first field).

   cut -d ':' -f 1 /etc/passwd
   

8. Extracting Hostnames from grep Output:

   • Extract the hostnames from the output of a grep command.

   grep "Host:" log.txt | cut -d ' ' -f 2
   

9. Extracting IP Addresses from Log Files:

   • Extract IP addresses from log files (assuming IPs are space-separated in the second field).

   cut -d ' ' -f 2 access.log
   

10. Extracting Fields with Variable-Length Delimiters:

    • Extract the third field from lines with variable-length delimiters (using a comma or colon).

    cut -d ',' -f 3 data.txt
    cut -d ':' -f 3 data.txt
    

cut is a simple yet versatile command for extracting specific columns or characters from text data. It is commonly used in combination with other Unix utilities such as grep, awk, and sort to process and manipulate structured data efficiently.