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chmod 4755 rwsr-xr-x
chmod 4755 (rwsr-xr-x): the owner can read, write and execute; group and others can read and execute, with setuid. Commands, umask and security notes.
- Octal
4755- Symbolic
rwsr-xr-x- ls -l shows
-rwsr-xr-xfor a file,drwsr-xr-xfor a directory- chmod letters
u=rwxs,g=rx,o=rx
Who can do what
| Class | Digit | Bits | On a file | On a directory |
|---|---|---|---|---|
| Owner (u) | 7 | rwx | Read the contents, change or empty the contents and run it as a program | List the names inside, create, delete and rename entries and enter it and open files by name |
| Group (g) | 5 | r-x | Read the contents and run it as a program | List the names inside and enter it and open files by name |
| Others (o) | 5 | r-x | Read the contents and run it as a program | List the names inside and enter it and open files by name |
Each digit adds read (4), write (2) and execute (1). Deleting or renaming a file depends on write and execute on the directory that holds it, not on the file's own mode. Root bypasses read and write checks.
Special bits
- Setuid (4)
- On a program: it runs with the file owner's user ID, whoever starts it. Shown as s in the owner's execute place. Linux ignores it on scripts and directories.
Typical uses
Setuid plus 755: the program runs with its owner's privileges, whoever starts it. /usr/bin/passwd is -rwsr-xr-x root, which is how an ordinary user can update /etc/shadow. Linux ignores setuid on scripts and on directories.
Security notes
- Setuid programs owned by root are a classic privilege escalation route if they have a bug. List them with find / -perm -4000 -type f and keep the set small.
- Only the owner (and root) can change it; the group and others are limited to the read and execute rights shown above, which is the usual safe pattern for shared files and directories.
- Every account on the machine can read it, so keep passwords, keys and tokens out of anything with this mode.
Commands
- Numeric
- Symbolic
- Check the result
The umask that gives 4755
No umask produces 4755: a umask only removes bits from 666 for files and 777 for directories, and it never sets setuid, setgid or the sticky bit. Set it with chmod after creating the file or directory.
Other common modes
400r--------, private keys that must not change440r--r-----, read-only config such as sudoers444r--r--r--, read-only files for everyone500r-x------, private read-only scripts550r-xr-x---, read-only programs shared with a group555r-xr-xr-x, read-only programs and directories600rw-------, SSH keys and private files640rw-r-----, config files and logs read by a group644rw-r--r--, web files and normal documents655rw-r-xr-x, a likely typo for 755660rw-rw----, files shared by a group664rw-rw-r--, team files under umask 002666rw-rw-rw-, device files like /dev/null700rwx------, ~/.ssh and private directories710rwx--x---, directories a group may pass through711rwx--x--x, home directories on shared hosting750rwxr-x---, home and app directories for a group755rwxr-xr-x, directories, programs and scripts770rwxrwx---, shared team directories775rwxrwxr-x, shared directories under umask 002777rwxrwxrwx, nothing, almost always a mistake1777rwxrwxrwt, shared temporary directories like /tmp2755rwxr-sr-x, setgid programs and group directories2775rwxrwsr-x, shared team directories that keep a group4755rwsr-xr-x, setuid programs like passwd