What is chmod?

    October 8, 2019

    Chmod refers to change mode, a command for changing file access permissions in a UNIX-based operating system.

    Purpose

    The chmod, or change mode, command allows an administrator to set or modify a file’s permissions. Every UNIX file has an owner user and an owner group attached to it, and every file has permissions associated with it. The permissions are as follows: read, write, or execute.

    Application

    UNIX systems have many users. In this context, a user may refer to an individual or a system operation. UNIX identifies each user with a UID, and users may be organized into groups.

    The syntax of the chmod command is:
    chmod mode file
    Example:
    chmod 720 readme.txt

    Each number in the mode parameter represents the permissions for a user or group of users:

    • The first number represents the file’s owner
    • The second number represents the file’s group
    • The third number represents everyone else

    Table 1 shows the eight numbers that can be used within the chmod parameter. The rwx column specifies read, write, and excecute access, offering a binary value for each operation. A "1" means "yes," a "0" means "no." If rwx reads 110, then that permission may read and write, but not excecute.

    # Permission rwx
    0

     none

    000
    1  execute only  001
    2 write only  010
    3  write and execute 011
    4  read only  100
    5  read and execute  101
    6  read and write  110
    7  read, write, and excecute  111

    Table 1. chmod mode parameters.

    For example, if you set your directory permissions to 720, then your permissions would function as follows:

    • The file’s owner may read, write, and execute the file.
    • The file’s group may only write the file.
    • All others cannot access the file.

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