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What is CHMOD?
Short for change mode, chmod is a command that changes the access permissions of files or directories in order to read, write or execute files. The mode number consists of three octal digits, representing the access allowed for yourself, for your group (other users set-up on your account), and for everyone else. The value of each digit represents the type of access that is allowed.
4 - ability to read the file
2 - ability to write the file
1 - ability to execute the file
If you wanted to represent the ability to read and write, you would add 4 and 2 together. For example, if you had a Perl script for your website, you might make the mode 755 so that you could read/write/execute (4 + 2 + 1 = 7) the file, whilst other users could only read/execute (4 + 1 = 5) the file. You may give a standard HMTL file, the mode 644 so that you can read and write to it, whilst others can only read it. You would usually make a directory executable, so a directory that others can read would have a mode of 755.