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See this page as a slide show

Permissions

CS155 Permissions

Permissions, Ownership, and ls -l

% date >now
% mkdir zulu
% ls -l
total 4
-rw------- 1 cs155 class 29 Nov 21 09:29 now
drwx------ 2 cs155 class 40 Nov 21 09:29 zulu

• Every file and directory in the file system is owned by some user and belongs in some group.
• Generally, a file is owned by whoever created it and belongs to that user’s default group.

Permissions in Detail

d rwx rwx rwx

d	rwx	rwx	rwx
directory or file	user	group	other

• First column: d means a directory, - means a file.
• Next three: permissions for the user (owner) of the file
• Next three: permissions for the group (similar people)
• Last three: permissions for others (everybody else)

The permissions can be different for user, group and other (everyone else). Typically, the user gets the most permissions, and others get very little.

Permissions: What do they mean?

• r: gives permission to read a a file or directory
• w: gives you permission to write a file or directory
• x: gives you permission to execute (run) a file or cd into a directory

Note that w for a directory means that you can change the directory, not the files it contains. Changing the files underneath it depends on their w bits.

Permissions: Examples

• drwxr-x---
  • owner: rwx: list, modify (add or remove files), and cd
  • group: r-x: list and cd
  • other: ---: no access
• -r--r--r--
  • a file that can only be read
• -rw-rw-r--
  • a file that can be read by everyone but only written by the owner and group members

chmod: Changing Permisions

• chmod : change mode
• Often we will want to grant or restrict access of certain files or directories. We can do this in two ways:
  • We can add or subtract permissions for the user, group, and others.
  • We can also set all permissions at once.

chmod: use

chmod mode[,mode]… file…

mode is formed by combining:

• level: u, g, or o (or combination thereof), or a for “all”
• modify symbol: +, -, or =
• permission: r, w, x
• Put commas between the modes—no spaces here!

chmod ug+wx gives user and group write and execute permissions. It doesn’t change the other permissions.

chmod: examples

chmod ugo=r chewbacca

give everyone permission only to read chewbacca. (-r--r--r--)

chmod go+xw darth

add group and other execute and write access to darth. (-????wx?wx)

chmod o-rwx han

take away read, write, and execute access from users not in group or user. (-??????---)

chmod u=rwx luke

give the user read, write, and execute access. (-rwx??????)

chmod: examples

% touch chewbacca darth luke han
% ls -l
total 0
-rw------- 1 cs155 class 0 Nov 21 09:29 chewbacca
-rw------- 1 cs155 class 0 Nov 21 09:29 darth
-rw------- 1 cs155 class 0 Nov 21 09:29 han
-rw------- 1 cs155 class 0 Nov 21 09:29 luke
% chmod ugo=r chewbacca
% chmod go+xw darth
% chmod o-rwx han
% chmod u=rwx luke
% ls -l
total 0
-r--r--r-- 1 cs155 class 0 Nov 21 09:29 chewbacca
-rw--wx-wx 1 cs155 class 0 Nov 21 09:29 darth
-rw------- 1 cs155 class 0 Nov 21 09:29 han
-rwx------ 1 cs155 class 0 Nov 21 09:29 luke

chmod: more examples

Command	Result
chmod a=r,go+w ackbar	-r--rw-rw-
chmod a=rwx,g-w,o-x ewok	-rwxr-xrw-
chmod a=rw,go-w,o-r yoda	-rw-r-----
chmod u=rw boba	-rw-??????
chmod a=rw,go= lando	-rw-------
chmod og=r,u=rw r2d2	-rw-r--r--

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