Question 1

What permission should SSH keys have?

Accepted Answer

Private keys (~/.ssh/id_rsa) must be chmod 600 (owner read/write only). The .ssh directory must be 700. Public keys can be 644. SSH will refuse to use keys with more permissive settings.

Question 2

What does the execute permission mean for directories?

Accepted Answer

For directories, execute (x) means permission to enter the directory (cd into it) and access files within it. Without execute on a directory, you cannot access any files inside even if you have read permission on those files.

Question 3

What is the difference between 755 and 644?

Accepted Answer

755 (rwxr-xr-x) includes execute permission — use for scripts, binaries, and directories. 644 (rw-r--r--) has no execute — use for regular data files like .txt, .html, .json. The key difference is whether the file can be run as a program.

Command	Description
chmod 777 file	Everyone can read, write, and execute (dangerous!) `rwxrwxrwx`
chmod 755 file	Owner: full access. Group/Others: read and execute `rwxr-xr-x — scripts, directories`
chmod 750 file	Owner: full access. Group: read/execute. Others: none `rwxr-x--- — group-shared executables`
chmod 700 file	Owner: full access. Group/Others: no access `rwx------ — private scripts`
chmod 644 file	Owner: read/write. Group/Others: read only `rw-r--r-- — most regular files`
chmod 640 file	Owner: read/write. Group: read. Others: none `rw-r----- — config files`
chmod 600 file	Owner: read/write. Group/Others: no access `rw------- — SSH keys, secrets`
chmod 400 file	Owner: read only. Group/Others: no access `r-------- — read-only secrets`

Command	Description
chmod +x file	Add execute permission for all
chmod u+x file	Add execute permission for owner only
chmod g+w file	Add write permission for group
chmod o-r file	Remove read permission for others
chmod -R 755 dir/	Apply permissions recursively to directory

Command	Description
4 = read (r)	Permission to read file contents or list directory
2 = write (w)	Permission to modify file or create/delete in directory

chmod Permissions Cheatsheet

Common Patterns

Symbolic Mode

Number Reference

Frequently Asked Questions

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