>> chmod

chmod

chmod is abbreviation of "change mode" , used to change the permission of files and folders.  There are three USERs who are granted permission or revoked permission. They are "USER or the OWNER" , "GROUP" , "OTHER". There are three permissions , "READ", "WRITE" , "EXECUTE". 

The READ, WRITE and EXECUTE are represented by the letters " r ",  " w ", " x " .

r = READ a file or a directory

w = WRITE to a file or a directory

x = EXECUTE a file or a directory

Permission to each member (OWNER or GROUP or OTHER) can be rwx or rw or wx or rx or r or w or x .

In numerical representation : r = 4 , w = 2 , x =1 . 

So,

rwx = 7 which is 4+2+1

rw  = 6 which is 4+2

wx = 3 which is 2+1

rx = 5  which is 4+1

r = 4

w = 2

x = 1

Syntax:

chmod  OWNERPermissionGROUPPermissionOTHERPermission  fileName

which is:

chmod  rwxrwxrwx  fileName

Example:

chmod 777 file1 = OWNER,GROUP,OTHER can READ, WRITE and EXECUTE the file1

chmod 400 file1 = OWNER can READ the file1 but OTHER members have no access.

chmod 742 file1 = OWNER can READ/WRITE/EXECUTE , GROUP can READ only, OTHER can WRITE only.

chmod 124 file1 = OWNER can EXECUTE only, GROUP can WRITE only and OTHER can READ only.

chmod 567 file1 = OWNER can READ/EXECUTE, GROUP can READ/ WRITE , OTHER can READ/WRITE/EXECUTE.

chmod 004 file1 = OWNER and GROUP have no access of anything, OTHER can READ the file1

and so on.

There is another way of granting or revoking permissions:

+ add access/permission

- remove access/permission

= set access/permission

chmod  a+rwx  file1  === add READ/WRITE/EXECUTE access to all(OWNER/GROUP/OTHER) on the file1.

chmod a-x file1  === remove EXECUTE access from all on the file1

chmod u=rwx, g=rw, o=r   file1  === set READ/WRITE/EXECUTE access to USER, READ/WRITE access to GROUP , READ only access to OTHER.

chmod u-wx,g-w,o=   file1   === remove WRITE/EXECUTE access from USER , remove WRITE access from GROUP, set no access to OTHER.

chmod  u+w, g-w   file1  ===  add WRITE access to USER , remove WRITE access from GROUP.

chmod  ug +rw  file1  === add READ/WRITE access to USER and GROUP.

chmod  u=x, go = rw  file1, file2, file3  ===  set EXECUTE access to USER, READ/WRITE access to GROUP and OTHER to file1 , file2 and file3.

and so on.