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Chmod 4755

chmod 4755 sets the permissions to rwsr-xr-x: it is 755 with setuid, so the program runs as its owner. Here is exactly what that allows on files and directories, when to use it, and the commands to apply it.

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ClassreadwriteexecuteValue
owner7
group5
others5
Common:
$ chmod 4755 file.sh
$ chmod u=rwxs,g=rx,o=rx file.sh
  • • The owner can read, write and execute.
  • • The group can read and execute.
  • • Everyone else can read and execute.
  • • Setuid: the file runs with the owner’s privileges.

What chmod 4755 means

WhoDigitLettersOn a fileOn a directory
owner 7 rwx Can read, modify and run Can list files, create, delete and rename files and enter it and open files
group 5 r-x Can read and run Can list files and enter it and open files
others 5 r-x Can read and run Can list files and enter it and open files

Each digit is the sum of read (4), write (2) and execute (1), and the leading digit sets the special bits: setuid (4), setgid (2) and sticky (1). In ls -l output it looks like this: 

$ ls -l
-rwsr-xr-x  1 alice  staff  1834 Oct 21 10:30 example.txt
  • The owner can read, write and execute.
  • The group can read and execute.
  • Everyone else can read and execute.
  • Setuid: the file runs with the owner’s privileges.

When to use 4755

  • passwd, sudo, ping (on older systems)
  • Rarely needed for your own programs

Setuid makes an executable run with its owner’s privileges, so a root-owned 4755 binary runs as root for every user. That is how passwd can update /etc/shadow. Every setuid binary is a potential privilege escalation, so audit them with find / -perm -4000 -type f 2>/dev/null. Linux ignores setuid on scripts.

Is chmod 4755 safe?

Only for small, carefully audited programs. Any bug in a setuid program can let a user gain its owner’s privileges.

Commands

# One file
chmod 4755 file

# Same thing in symbolic form
chmod u=rwxs,g=rx,o=rx file

# Every file under a tree (leaves directories alone)
find /path/to/dir -type f -exec chmod 4755 {} +

# Check the result
stat -c '%a %A %n' file    # Linux
stat -f '%Lp %Sp %N' file  # macOS

Avoid chmod -R 4755 on a whole tree: it gives files and directories the same mode. The find form above changes only files.

Other chmod values

777 · 775 · 770 · 755 · 751 · 750 · 744 · 711 · 700 · 666 · 664 · 660 · 644 · 640 · 600 · 555 · 500 · 444 · 400 · 000 · 1777 · 2775

Need a different combination? Use the Chmod Calculator.

Sources

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Frequently asked questions

What does chmod 4755 mean?

chmod 4755 sets the permissions to rwsr-xr-x. The owner can read, write and execute. The group can read and execute. Everyone else can read and execute. Setuid: the file runs with the owner’s privileges.

Is chmod 4755 safe?

Only for small, carefully audited programs. Any bug in a setuid program can let a user gain its owner’s privileges.