File Permissions

File permissions, also known as access modes, determine who can access and modify files. On the command line you can list files along with their permissions with ls -l, and you can change file permissions using the chmod command.

In a directory listing, permissions appear by default as a sequence of ten characters, something like drwx------ or -rw-rw-r--, for instance.

Here's an example directory listing -- the first column shows the file permissions:

    $ ls -l
    drwxrwxr-x  4 bart bart     4096 Oct 27 21:41 ./
    drwxr-xr-x 23 bart bart     4096 Oct 27 21:30 ../
    -rw-rw-r--  1 bart friends     0 Oct 15 22:01 foo
    drwxrwxr-x  2 bart bart     4096 Oct 27 21:41 public/
    drwx------  2 bart bart     4096 Oct 27 21:40 secrets/
    -rw-rw----  1 bart friends     7 Oct 15 22:01 wibble

The very first character shows whether or not the file is a directory. (That first character has other uses too, but we won't cover that here.) The next nine characters represent the actual file permissions. If you split those nine characters into three 3-character sequences, you get the permissions for 1) the file owner, 2) group members, and 3) everyone else.

         g   o
     u   r   t
     s   o   h
     e   u   e
     r   p   r

    rwx rwx rwx   <-- read, write, execute (or directory access)
    421 421 421       these equate to the octal bits 4, 2, and 1 respectively

The characters r, w, and x represent read access, write access, and permission to execute the file as a program (or to grant directory access if the file is a directory).

In the example directory listing above:

foo is a file that can be read by anyone, but only written to by bart or members of the friends group.
public is a directory that anyone can access, but only bart can modify.
secrets is a directory that only bart can access.
wibble is a file that bart and any friends group member can acess and modify, but nobody else can access at all.

To view the octal values:

    $ stat -c "%a %n" -- *
    664 foo
    775 public
    700 secrets
    660 wibble

(Be aware that system administrators can access files regardless of file permissions. Permissions can help manage files, but aren't about absolute privacy.)

Using `chmod` to modify file permissions

The chmod command understands two equivalent access mode formats:

    chmod u=rw,g=,o= FILENAME(s)     chmod u=rwx,g=,o= FILENAME(s)
    chmod 600 FILENAME(s)            chmod 700 FILENAME(s)

    chmod u=rw,g=r,o= FILENAME(s)    chmod u=rwx,g=rx,o=rx FILENAME(s)
    chmod 640 FILENAME(s)            chmod 755 FILENAME(s)

    chmod r=rw,g=rw,o=r FILENAME(s)
    chmod 664 FILENAME(s)

Those octal values can be derived by adding the relevant octal bits. Some common examples:

    rw- --- ---    rw- r-- ---    rw- rw- r--    rwx --- ---    rwx r-x r-x
    42- --- ---    42- 4-- ---    42- 42- 4--    421 --- ---    421 4-1 4-1
     6   0   0      6   4   0      6   6   4      7   0   0      7   5   5

See the chmod manual page (man chmod) for more information. The related chown and chgrp commands can be used to manage file ownership.

last compiled: 2021-06-04 12:34:03.178788

File Permissions

Using chmod to modify file permissions

Using `chmod` to modify file permissions