Changing file permissions

The chmod command is used to change the permissions for a file. Only the owner of a file or the superuser can modify its permissions. The format of the chmod command is

  chmod ugo=rwxs

where

• the letters u, g, and o specify the category of user: user (owner), group, and other respectively. Omit any of these to which the permission is not intended to apply. You may use a to mean all, i.e. equivalent to ugo.
• the letters r, w, x, and s specify the category of permission: read, write, execute, and set-id respectively (replace s by t if the permission applies to other). Omit any of these permissions which are not being granted.
• Use = to set file's permissions to exactly the ones that are specified.
• Replace = by + to add the specified permissions to those the file already has.
• Replace = by - to remove the specified permissions from those the file currently has.

For instance,

  chmod a+x foo.pl

adds execution permission for all three categories of users to file foo.pl.

  chmod o-rwx private

denies all access to directory private to users other than its owner or members of its group.

For convenience, those who know the octal number system can use a numeric code to specify permissions. To define the octal number, each of the three rwx fields corresponds to one digit. If r is present, the digit contains a 4, if w is present, the digit contains a 2, and if x is present the digit contains a 1. These three numbers are summed to produce the octal digit. For instance

  chmod 750 bar

sets the permission of bar to rwxr-x---. (The octal method can also be used to set the set-id permissions. See the chmod man page for details.)