change permissions for file

chmod is the command for changing permission of files.

4 - read (r)
2- write (w)
1- execute (x)

rwx-rwx-rwx are the permission bits.

first group rwx belongs to owner , 2nd group rwx belongs to users who belong to owners group.
third group of rwx belongs to other users.

chmod  755 file.txt

here 7  =  4+2+1 rwx permission to owner of file
5 = 4+1  r+x permission to user who belongs to group of owner
5= 4+1 read+execute to others