Skip to main content

File Permissions in Linux and Unix

 File system security (access rights)

In your home directory, type
% ls -l (l for long listing!)

You will see that you now get lots of details about the contents of your directory, similar to the example below.

Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):
-rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1
In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory: otherwise - will be the starting symbol of the string.
The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3.
  • The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example); 
  • the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example);
  • the rightmost group gives the permissions for all others.
The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.

Access rights on files.

  • r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file 
  • w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file 
  • x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate

Access rights on directories.

  • r allows users to list files in the directory;
  • w means that users may delete files from the directory or move files into it;
  • x means the right to access files in the directory. This implies that you may read files in the directory provided you have read permission on the individual files.
So, in order to read a file, you must have execute permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.

Some examples

a file that everyone can read, write and execute (and delete).
a file that only the owner can read and write - no-one else 
can read or write and no-one has execution rights (e.g. your 
mailbox file).

 Changing access rights

chmod (changing a file mode)

Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows

write (and delete)
execute (and access directory)
add permission
take away permission

For example, to remove read write and execute permissions on the file biglist for the group and others, type
% chmod go-rwx biglist
This will leave the other permissions unaffected.
To give read and write permissions on the file biglist to all,
% chmod a+rw biglist


Popular posts from this blog

CentOS / Redhat : Configure CentOS as a Software Router with two interfaces

Linux can be easily configured to share an internet connection using iptables. All you need to have is, two network interface cards as follows: a) Your internal (LAN) network connected via eth0 with static ip address b) Your external WAN) network is connected via eth1 with static ip address  ( public IP provided by ISP ) Please note that interface eth1 may have public IP address or IP assigned by ISP. eth1 may be connected to a dedicated DSL / ADSL / WAN / Cable router: Step # 1: Enable Packet Forwarding Login as the root user. Open /etc/sysctl.conf file # vi /etc/sysctl.conf Add the following line to enable packet forwarding for IPv4: net.ipv4.conf.default.forwarding=1 Save and close the file. Restart networking: # service network restart Step # 2: Enable IP masquerading In Linux networking, Network Address Translation (NAT) or Network Masquerading (IP Masquerading) is a technique of transceivin

Shell Script: Find Number Of Arguments Passed

Many times , when we create shell scripts we try to do repetitive tasks through functions. Some functions take arguments & we have to check the no. of arguments that are passed to it. Each bash shell function has the following set of shell variables: [a] All function parameters or arguments can be accessed via  $1, $2, $3,..., $N . [b]  $*  or  $@  holds all parameters or arguments passed to the function. [c]  $#  holds the number of positional parameters passed to the function. [d] An array variable called  FUNCNAME  ontains the names of all shell functions currently in the execution call stack. Example Create a shell script as follows: #!/bin/bash # Purpose: Demo bash function # -----------------------------   ## Define a function called test() test(){   echo "Function name:  ${FUNCNAME}"   echo "The number of positional parameter : $#"   echo "All parameters or arguments passed to the function: '$@'"   e

Linux Find Command: Find Files Modified On Specific Date

There are many situations in which we have to find out  all files that have been modified on a specific date  using find command under Linux. There are two ways to list files in given directory modified after given date of the current year. The latest version of GNU/find command use the following syntax: Syntax GNU/find latest version: find /path/to/dir -newermt "date" find /path/to/dir -newermt "May 13" find /path/to/dir -newermt "yyyy-mm-dd" ## List all files modified on given date find /path/to/dir -newermt yyyy-mm-dd ! -newermt yyyy-mm-dd -ls ### print all *.sh ### find /path/to/dir -newermt "yyyy-mm-dd" -print -type f -iname "*.sh" The other way of doing this works on the versions of find before v4.3.3: touch -t 02010000 /tmp/timestamp find /usr -newer /tmp/timestamp then we can remove the reference file: rm -f /tmp/stamp$$ To  find out all Shell Script files  (*.sh) in /home/linux/scripts that have been modifie