400 linux permission

The permission 400 in Linux refers to file permissions that you can set using the chmod command. Here’s what it means:

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Breakdown of 400 Permission

Permissions in Linux are represented by three digits:

• The first digit is for the owner (user).
• The second digit is for the group.
• The third digit is for others.

Each digit can be the sum of:

• 4 = read (r)
• 2 = write (w)
• 1 = execute (x)

So:

• 400 = 4 (read for owner) + 0 (no permissions for group) + 0 (no permissions for others)

Symbolic representation:
-r--------

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Meaning

• Owner: Can read the file, but cannot write or execute.
• Group: Cannot read, write, or execute.
• Others: Cannot read, write, or execute.

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When to Use 400 Permission?

• Sensitive files: Use for files that should only be read by the owner (e.g., private keys like id_rsa for SSH).

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How to Set 400 Permission

chmod 400 filename

Example:

chmod 400 id_rsa

This makes id_rsa readable only by you (the file’s owner), which is often required by SSH.

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Summary Table

User	Read	Write	Execute
Owner	✔️	❌	❌
Group	❌	❌	❌
Others	❌	❌	❌

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Bottom line:

• 400 = Only the file’s owner can read it.
• Typical for sensitive, private files (like SSH keys).

Let me know if you want to understand other permission numbers!