command chmod change mode

chmod is used to change the permissions of files or directories

chmod options permissions filename

        4 stands for `read`
        2 stands for `write`
        1 stands for `execute`
        0 stands for `no permission`

chmod 644 filename
1. owner can read and write
2. group can read only
3. others can read only

read and write by everyone

      $ chmod 666 filename
      $ chmod a=rw filename