chmod 1777: sticky bit (rwxrwxrwt)

Answer

chmod 1777 is 777 plus the sticky bit (the leading 1). Everyone can create files in the directory, but a file can only be deleted or renamed by its owner, the directory's owner, or root. ls -l shows it as a t at the end: rwxrwxrwt. This is the mode of /tmp.

chmod 1777 = rwxrwxrwt: everyone full access, plus sticky.

	Read 4	Write 2	Execute 1	Digit
Owner (user)	Owner (user) rr	Owner (user) ww	Owner (user) xx	7
Group	Group rr	Group ww	Group xx	7
Others	Others rr	Others ww	Others xx	7
Special	setuidsetuid	setgidsetgid	stickysticky	1

7
Owner (user): 7 = 4 read + 2 write + 1 execute
7
Group: 7 = 4 read + 2 write + 1 execute
7
Others: 7 = 4 read + 2 write + 1 execute
1
Special bits: 1 sticky: in a shared directory, only a file's owner (or root) can delete or rename it

World-writable with the sticky bit: the standard mode for shared directories such as /tmp. Anyone can create files, but only a file's owner can delete or rename it. On a regular file it is still a hole.

On a file

Owner (user) read it, change it and run it as a program
Group read it, change it and run it as a program
Others read it, change it and run it as a program

On a directory

Owner (user) list the names inside, enter it and open files by name, create and rename and delete entries
Group list the names inside, enter it and open files by name, create and rename and delete entries
Others list the names inside, enter it and open files by name, create and rename and delete entries

chmod 1777 filechmod u=rwx,g=rwx,o=rwxt filels -l → -rwxrwxrwt

What to know

Without the sticky bit, write permission on a directory lets a user remove or rename any entry in it, whoever owns the file. On a shared directory like /tmp, that would let any user delete or swap out another user's temporary files. The sticky bit removes that ability.

On Linux, /tmp, /var/tmp and /dev/shm are normally 1777. The bit only matters on directories; Linux ignores it on regular files.

Where it fits

/tmp, /var/tmp and other world-writable scratch directories
Shared drop or upload directories where users must not delete each other's files (often as 1770 or 3775 instead of 1777)

Gotchas

It does not protect file contents: a world-writable file inside /tmp can still be edited by anyone. Programs should create temp files with mktemp, which uses mode 600.
A capital T (for example 1776) means sticky is set but others lack execute, so they cannot use the directory anyway.
Because 1777 contains world write, tools that flag world-writable paths will list it. On a /tmp-style directory with the sticky bit, that is expected.
If /tmp loses its sticky bit (for example after restoring it from a backup without modes, or a mistaken chmod 777 /tmp), restore it with chmod 1777 /tmp.

Related explainers

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