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Permissions.adoc

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Permissions

Table of Contents

UNIX OSS are multi-user: more than one user can be operating the computer at the same time. For example if your computer is connected to a network, or the internet, remote users can log in using a secure shell.

To protect different users from each other or prevent one user crashing the computer, prevent users interfering with files belonging to somone else:

chmod modify access rights sudo - temporry access as root user (system administrator) su - temporarily become the superuser chown - change file ownership chgrp - change a file’s goup ownership

File Permissions (ls -l)

$ ls -l /bin/bash
-r-xr-xr-x  1 root  wheel  618416 May  4 00:49 /bin/bash

chmod

  • chmod changes the mode (permissions) of a file or directory

  • you must specify the desired mode (permission settings) and the file or files that you wish to modify

run:

$ man chmod

look at the first lising under SYNOPSIS:

chmod [-fv] [-R [-H | -L | -P]] mode file …​

remember that everything in brackets is optional.

Instead let’s focus on the mode.

Remeber, man pages use less, to find information about mode, let’s do a search using the less commands. But first, to get help type 'h'. Now scroll down utnil you find information about searching.

SEARCHING
  /pattern          *  Search forward for (N-th
) matching line.

In the first line underneath SEARCHING it suggests tha we can look use /pattern to search forward for mathing lines

type q to leave the list help page. now were back in the man page for chmod. Enter the forward slash and type

/mode

then hit enter and scroll until you find some useful information.

Under DIAGNOSTICS it explains that modes can be absolute or symbolic.

For this case, symbolic modes will be easier to work with. keep scrolling.

The symbolic mode is described by the
     following grammar:
 mode         ::= clause [, clause ...]
           clause       ::= [who ...] [action ...] action
           action       ::= op [perm ...]
           who          ::= a | u | g | o
           op           ::= + | - | =
           perm         ::= r | s | t | w | x |X | u | g | o
 The who symbols ``u'', ``g'', and
     ``o'' specify the user, group, and
     other parts of the mode bits, respec-
     tively.  The who symbol ``a'' is equivalent to ``ugo''.

Scroll down until you find information about the mode

How would you use the chmod command to make sure that anyone in any group would be able to read the contents of a file you create. Also, how would you make a file executable (so it can be run as a program).

file accessible to read by anyone chmod a+r filename

file executable just for you: chmod u+x filename

octal notation

think of permssion settings as a series of bits, this is how the computer thinks about them. Here’s how it works:

rwx rwx rwx = 111 111 111

rw- rw- rw- rw- = 110 110 110

r-- r-- r-- = 100 100 100

and so on...

rwx = 111 in binary = 7
rw- = 110 in binary = 6
r-x = 101 in binary = 5
r-- = 100 in binary = 4

Each of the three sets of permissions can be represented by a single digit for the owner, group and other.

So, if we wanted to set read and write priveleges for the owner but no permissions for group and other, type:

$ chmod 600 some_file

reference

777 - rwxrwxrwxrwx no restrictions. Anybody can do anything.

755 (rwxr-xr-x) - file owner has no restrictions, group and other can only read and execute.

700 - rwx------ 666 r-xr-xr-x 644 rw-r—​r-- 600 - rw--------

Directory Permissions

this time r,w and x attributes have different meaning

r - allows content of directory to be listed if the x attribute is also set x- allows files within directory to be created, deleted or renamed if the xattribute is also set x - allows a directory to be entered (i.e. cd dir)

Useful settings for directories:

777 - rwxrwxrwx anyone can list files, create new files and delete old ones in the directory. Generally not a good setting.

755 - rwxr-xr-x directory owner has full access all others may list the directory but cannot create files nor delete old ones. common setting for directories that you’ll share with others.

700 rwx------ the directory owner has full access. no one can list, make, or delete files within the directory. essentially a private directory.

Becoming the Superuser for a Short While (su)

  • often necessary to become the superuser to perform imoprtant system admin tasks

Warning
 don’t stay logged in too long. can mess things up.

su - chot for subtitue user, can be used when you need superuser access for small number of tasks:

$ su
Password:
su: Sorry
$ sudo passwd
Changing password for root.
New password:
Retype new password:
$ su
Password:
sh-3.2#

to exit:

sh-3.2# exit

sudo

with sudo one or more users are granted superuser priveleges on an as need basis

to run command as superuser, using sudo:

$ sudo some_command
Password:
$
Note
 password is user password, not superuser

changing file ownership

change file owner using chown command

to change owner of file from me to you:

$ su
Password:
# chown you some_file
# exit
$

chown works same on directories as files

Note
 to change owner of file, must be superuser.

Changing Group Ownership (chgrp)

$ chgrp new_group som_file
Note
 must be owner of file or directory to perform chgrp

Common Error Message

n error message which Linux users might encounter: "Permission denied."

  • Trying to read or alter a file which you have no right to  Trying to delete a file or directory which you have no right to  Trying to create a file in an area which you have no right to

try:

$ mkdir /etc/wibble
mkdir: /etc/wibble: Permission denied

This is because /etc is owned by the systems administrators account (known as ‚root‛) and root has not given write permission to this area.