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chmod 755 gives the owner read, write and execute permission (7 = 4 + 2 + 1) and gives the group and everyone else read and execute (5 = 4 + 1). In ls -l it shows as rwxr-xr-x. Use 755 for directories and scripts, 644 (rw-r--r--) for normal files, and change the owner with chown, for example sudo chown alex:www-data file.

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Before you start

  • Connect with SSH. You can change permissions on your own files; for other files you need sudo (root can leave it out).
  • Take a backup or snapshot before you change permissions recursively on a large folder.

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How to read Linux permissions

Run ls -l:
Each letter has a number. Add them up for each of the three positions:

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What do chmod 755, 644, 600 and 777 mean?

Donā€™t ā€œfixā€ a permission error with chmod 777. It lets every user and every process on the server change or replace the file, which is a common way websites get hacked. Find out which user needs access and give only that user or group the rights it needs.

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How to change permissions with chmod

1

Set permissions with numbers

2

Or change one part with letters

u = owner, g = group, o = others, a = all. + adds, - removes, = sets exactly.
3

Check the result

Expected output:

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How to set different permissions for files and directories

chmod -R 755 makes every file executable, which you rarely want. Use find to treat them separately, for example for a website:

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How to change the owner with chown

The web server user is www-data on Ubuntu and Debian, and nginx or apache on AlmaLinux and Rocky Linux, depending on which web server you installed.
Never run chown -R or chmod -R on /, /etc, /usr or /var as a whole. Changing system ownership breaks sudo, SSH and other services, and the only reliable fix is a restore from backup or a reinstall. Double-check the path before pressing Enter.

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Special permissions: setgid and sticky bit

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What is umask?

umask sets the default permissions for new files. The usual value 0022 makes new files 644 and new directories 755. Check yours with umask.

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How to check it worked

Test as the user who needs access, for example the web server user:
If the file prints, the permissions are right. Permission denied means the user is missing r on the file or x on one of the parent directories. namei -l /var/www/example.com/index.html shows the permissions of every directory in the path.

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Frequently asked questions

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What is the difference between chmod 755 and 644?

755 adds execute (x). Directories need x so you can open them, and scripts need it to run. Plain files such as HTML, images and config files donā€™t, so they use 644.

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What permissions should ~/.ssh have?

~/.ssh must be 700 and ~/.ssh/authorized_keys 600, owned by the user. SSH refuses keys when they are more open. See fix SSH ā€œPermission denied (publickey)ā€.

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What does chmod +x do?

It adds execute permission for the owner, group and others (limited by your umask), so you can run the file with ./script.sh.

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Why does chmod say ā€œOperation not permittedā€?

Only the owner or root can change a fileā€™s permissions. Use sudo, or check the owner with ls -l.

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Checked against the official GNU Coreutils manual: chmod, chown and file modes on 4 November 2026; commands tested on Ubuntu 24.04 LTS. Commands can differ on other distributions or releases.

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Last Updated: November 2026