head — view the beginning of a file

💬 "head command"

head displays the first N lines of a file. It is useful for checking the header of a formatted file.

Use it to quickly understand the structure before opening a large file.

• Concept: Prints the beginning of a file (10 lines by default).
• Left argument: Use -n number to specify the number of lines.
• Right argument: The target file.
• Multiple files: With multiple files, it shows the beginning of each file along with a per-file header.

Commonly used options

3 essential command combinations

• head -n 1 /var/log/nginx/access.log — Check the format with the first log line
• head -n 3 data.csv — Check the first 3 CSV header lines
• head -c 64 /opt/app/bin/app.bin | xxd | head — Check the binary signature

💬 "Give me an example from a security audit perspective"

Here are common situations on a production server, organized in order.

Step 1 · Check the format

Understand the structure with the first few lines of the file.

head -n 2 /etc/nginx/nginx.conf

Step 2 · Compare samples

Compare the beginning of several files.

head -n 2 /var/log/nginx/*.log

Step 3 · Check binary

View byte-level output if necessary.

head -c 32 /opt/app/bin/app | xxd

How it is used in practice

1. Check CSV columns

Situation — You need to check the column order of an uploaded CSV first.

head -n 3 /srv/import/orders.csv

Result — Check the column order and delimiter with the header and two sample lines.

Point — Checking the format with head before writing an import script reduces trial and error.

2. Check the first block of a config file

Situation — You want to quickly see only the first structure among several server blocks.

head -n 20 /etc/nginx/nginx.conf

Result — Quickly understand the global settings and include structure.

Point — Use head to grasp the structure before opening the full file, then inspect in detail with less.

Points to watch

• The default is 10 lines, so unless you specify the number of lines, you may not get enough information.
• If you set the number of lines too high, using head loses its purpose.
• If a CSV has fields containing line breaks, simple line counting will be off.

Actual output

This is the result of checking the header and sample lines of a CSV file.

!head command output: CSV header and samples

Production server

Source (HTML): https://games.aidebatehub.com/programs/linux/logs/head.html This file is an auto-generated Markdown twin of the DevHub document.