The chmod Linux command is used to change the access mode (aka file system permissions) of one or more files (or directories) only the owner or a privileged user may change the mode.

Chmod Command Usage

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chmod [options] mode files

The mode can be specified by the name or the UID/ GID number, if you do not specify who then the default option of a (all) will be used. You may only use one opcode and multiple modes are separated by commas.

Situations when the chmod command would be used

• To change file permission on Linux / Unix based operating systems
• Set file as read / write / execute on Linux or Unix
• To set the sticky bit in Linux using chmod
• Set group / user file system permissions in Linux

chmod options

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-c, –changes

    Print info about files that are changed

-f, –silent, –quiet

    Do not notify user of files that chmod cannot change (stops chmod printing warnings on the console)

–help

    Prints the help message

-R, –recursive

    Traverse subdirectories recursively, apply changes (meaning chmod will change the file permissions of all the files & dir’s in the below directories)

–reference=filename

     Change the permissions to match those associated with filename (it will change the permissions t o match those of the specified file name

-v, –verbose

    Print info about each file, even if it has not been changed

 –version

    Prints / shows the chmod version

Who

• u User
• g Group
• o Other
• a All (the default option)

Opcode

• • Add permission
• • Remove permission
• = Assign permission (and remove permission of the unspecified fields).

Permission Options for Chmod

• r Read
• w Write
• x Execute
• s Set user (or group) ID
• t Sticky bit – used on directories to prevent removal of files by non-owners, also used to make sure permissions are taken from the parent directory
• u User’s present permission
• g Group’s present permission
• o Other’s present Permission

chmod permission number explained

An alternative option to specifying the above is to use the 3 digit octal number method (e.g 755), remember the following:

• The first chmod number is the owner permission
• The second chmod number is the group permission
• The third chmod number is the other’s permission

What each chmod number means: * 4 = Read * 2 = Write * 1 = Execute

There mat be a fourth number on the end this has the following meaning:

• 4 = Set user ID on execution to grant permissions based on the files owner, not the user who created the process. So for example if you ran a file it would run as the user who created it, not your user
• 2 = Set group ID on execution to grant permissions based on the files owner, not the user who created the process.
• 1 = Set the sticky bit

chmod examples

Below are some real world examples of the chmod command. Add execute permissions to a file (note this will make it executable to everyone as I am not specifying user, group or other):

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chmod +x filename.sh

Add execute permissions to the user for a file:

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chmod u+x filename.sh

Set read, write, execute to everyone:

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chmod 777 filename.sh

Another way of doing this would be:

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chmod u=rwx,g=rwx,o=rwx filename.sh

chmod set read / write / execute for the user and read / execute for the group and execute only for others:

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chmod 751 filename.sh

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chmod u-rwx,g=rx,o=x filename.sh

chmod read / write / execute for the new owner only:

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chmod 700 filename.sh

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chmod u-rwx,g=rx,o=x filename.sh

chmod read / write / execute for the owner only:

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chmod 700 filename.sh

chmod read / write / execute for the owner & read / execute for the group and other (global):

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chmod 755 filename.sh

The following sets the UID to read / write and read, write , execute to the owner and read, execute to the group and other.

Common chmod file system numbers chart

You add the numbers together to get the correct file system permissions, for example 4 is read 2 is write so read / write is 6.

• 700 = chmod read, write, execute for the owner only
• 755 = chmod read, write, execute for the owner – read, execute for group and others
• 644 = chmod read, write by the owner – read only the group and other (world readable)
• 600 = chmod read, write by the owner only