How to Cut Sections of Line in File

The cut command in Linux is a text processing utility designed to extract specific sections (columns) from each line of files or input streams. It allows users to “cut out” selected portions of text based on byte position, character position, or field position (when using a delimiter).

This makes it particularly useful for working with structured text files like CSV files, logs with consistent formatting, or any text with predictable patterns.

Extract Fields by Delimiter (e.g., CSV)

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

• -d ',': Uses comma as the delimiter.
• -f 1,3: Selects the first and third fields (columns).

Extract a Single Column from a Space-Delimited File

cut -d ' ' -f 2 logfile.txt

• Splits lines by spaces and extracts the second field.

Extract Characters by Position

cut -c 1-5 names.txt

• -c 1-5: Displays the first 5 characters of each line.

Extract Bytes from a Binary/Text File

cut -b 10-20 file.bin

• -b 10-20: Shows bytes 10 to 20 of the file.

Split /etc/passwd by Colon (:)

cut -d ':' -f 1,6 /etc/passwd

• Extracts the username (1st field) and home directory (6th field).

Use cut with Command Output (e.g., ls -l)

ls -l | cut -d ' ' -f 5-

• Extracts file sizes and names from ls -l (fields 5 and onward).

Suppress Errors for Missing Delimiters

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

• -s: Skips lines without the delimiter (prevents error messages).

Invert Selection (Exclude Fields)

cut --complement -d ',' -f 2 data.csv

• Excludes the second field and displays all others.

Extract Fixed-Width Fields (No Delimiter)

cut -c 1-10,20-30 fixed_width.txt

• Selects characters 1–10 and 20–30 from each line.

Combine with grep and sort

grep "ERROR" app.log | cut -d ' ' -f 3 | sort -u

• Extracts the third field from error lines and sorts unique values.

Advanced Examples

Get a list of all shell users (with real shells)

grep -v "nologin\|false" /etc/passwd | cut -d: -f1,7

Extract columns from fixed-width data

cut -c1-10,15-25,30-40 fixed_width_data.txt

Find the top 5 largest directories (by item count)

find . -type d -name "*" | while read dir; do echo "$(ls -1 "$dir" | wc -l) $dir"; done | sort -rn | head -5 | cut -d' ' -f2-

Parse and extract specific fields from JSON (with helpers)

cat data.json | jq -r '.users[] | [.name, .email] | @csv' | cut -d, -f1

Key Notes:

• Delimiters: Use -d for columns (e.g., -d ',' for CSV).
• Field vs. Character Mode: -f selects fields, -c selects characters.
• Limitations: cut cannot handle multi-character delimiters (use awk for complex parsing).