Linux ‘chmod’ command on Ubuntu Server

About chmod

Changes the permission of a file.

Syntax

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

-c, –changes like verbose but report only when a change is made
–no-preserve-root do not treat `/’ specially (the default)
–preserve-root fail to operate recursively on `/’
-f, –silent, –quiet suppress most error messages
-v, verbose output a diagnostic for every file processed
–reference=RFILE use RFILE’s mode instead of MODE values
-R, –recursive change files and directories recursively
–help display this help and exit
–version output version information and exit

Permissions

u - User who owns the file.
g - Group that owns the file.
o - Other.
a - All.
r - Read the file.
w - Write or edit the file.
x - Execute or run the file as a program.

Numeric Permissions:

CHMOD can also to attributed by using Numeric Permissions:

400 read by owner
040 read by group
004 read by anybody (other)
200 write by owner
020 write by group
002 write by anybody
100 execute by owner
010 execute by group
001 execute by anybody

Examples

The above numeric permissions can be added to set a certain permission, for example, a common HTML file on a Unix server to be only viewed over the Internet would be:

chmod 644 test.html

This gives the file read/write by the owner and only read by everyone else (-rw-r--r--).
Files such as scripts that need to be executed need more permissions. Below is another example of a common permission given to scripts.

chmod 755 test.cgi

This would be the following 400+040+004+200+100+010+001 = 755 where you are giving all the rights except the capability for anyone to write to the file.cgi file(-rwxr-xr-x).

chmod 666 test.txt

Finally, another common CHMOD permission is 666, as shown below, which is read and write by everyone.
Additional information
Below is an example of how a file may be listed when typing ( ls -l ) at the prompt as well as information on how to interpret it.
-rw-rw-r– 1   swapnil 123   Nov 14 15:36   test.txt

rw rw- r– 1 swapnil 123 Nov 14 15:36 test.txt
File owner group everyone else links owner size mod date file name

Hope you find this tutorial helpful. Feel free to ask questions! Don’t forget to like or to leave a comment if its really help you.

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