chmod 660 rw-rw----

chmod 660 (rw-rw----): the owner can read and write, the group read and write, others do nothing. Typical use: files shared by a group. Commands and umask.

Octal: 660 (also written 0660)
Symbolic: rw-rw----
ls -l shows: -rw-rw---- for a file, drw-rw---- for a directory
chmod letters: u=rw,g=rw,o=

Who can do what

Class	Digit	Bits	On a file	On a directory
Owner (u)	6	`rw-`	Read the contents and change or empty the contents	List the names inside, nothing more with write alone (it needs execute to work) and see names only, not open or inspect the files
Group (g)	6	`rw-`	Read the contents and change or empty the contents	List the names inside, nothing more with write alone (it needs execute to work) and see names only, not open or inspect the files
Others (o)	0	`---`	Nothing	Nothing

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.

Typical uses

The owner and the group read and write, others get nothing. Used for device files such as disks (/dev/sda is 660 root:disk) and serial ports (group dialout), shared data files edited by a team, and sockets a service shares with its group.

Security notes

The group can write, so check who is in the group; adding a user to it gives them write access.

Commands

Numeric
chmod 660 file
Symbolic
chmod u=rw,g=rw,o= file
Check the result
stat -c '%a %A %n' file
Files only, recursively
find /path -type f -exec chmod 660 {} +

The umask that gives 660

umask 007: new files are created as 660 (and directories as 770)

Programs create files with 666 and directories with 777, and the umask removes bits from those. A umask never adds execute to a new file, and it cannot set setuid, setgid or the sticky bit.

Its directory counterpart is chmod 770 (rwxrwx---).

Other common modes

Work out any other mode in the chmod calculator