csplit — Split by pattern

Linux / Basic Linux commands / csplit.html

Splits a file at points defined by patterns or line numbers.

Basic Information

What is this command?

Unlike split, csplit cuts a file at points where a regex or line number occurs. It is useful for separating log sessions at each start marker.

Patterns use /regex/ syntax, with {n} for repetition and {*} to repeat to the end. If a pattern never matches, the section is not created, so test against real data first.

What problems does it solve?

• Separating logs at each session marker
• Splitting a file containing several documents
• Cutting sections at known line numbers
• When split cannot respect content boundaries

Key features

Pattern split

A new file starts at each /regex/ match.

• When to use: Splitting on a recurring delimiter
• What you get: Content boundaries are respected.
• Limitations: No match means no file is produced.

Repetition

{n} and {*} control how often to split.

• When to use: When the number of sections is known
• What you get: Prevents unwanted extra pieces.
• Limitations: A wrong count merges the final sections.

Output names

Prefix and format control the output names.

• When to use: When later steps rely on naming
• What you get: Script integration becomes easier.
• Limitations: Existing files can be overwritten.

Installation and usage

Check whether it is installed

csplit --version

sudo apt install coreutils # Debian/Ubuntu sudo dnf install coreutils # Fedora

Real-world examples

Split by session

Start a new file at each START line.

csplit app.log '/^START/' '{*}'

Split at a line

Split before line 100.

csplit notes.txt 100

Limit sections

Split only twice.

csplit app.log '/^START/' '{2}'