chmod

Change access permissions (file modes)

SYNTAX 
           chmod [-fhv] [-R [-H | -L | -P]] mode file ...

KEY
     -R	     Recurse: Change the mode of file hierarchies rooted in the files
	     instead of just the files themselves.

     -R -H      Follow symbolic links on the command line
	       (by default Symbolic links within the tree are not followed.)	       

     -R -L      All symbolic links are followed.

     -R -P      No symbolic links are followed. (default)
	     

     -f	     Do not display a diagnostic message if chmod could not modify the
	     mode for file.

     -h	     If the file is a symbolic link, change the mode of the link
	     itself rather than the file that the link points to.

     -v	     Verbose, show filenames as the mode is modified.

     -v -v   Very Verbose: display both old and new modes of the file
	     in both octal and symbolic notation.

chmod changes the permissions of each given file according to MODE, which can be either an octal number representing the bit pattern for the new permissions or a symbolic representation of changes to make, (+-= rwxXstugoa)

Numeric mode:

From one to four octal digits
Any omitted digits are assumed to be leading zeros.

The first digit = selects attributes for the set user ID (4) and set group ID (2) and save text image (1)S
The second digit = permissions for the user who owns the file: read (4), write (2), and execute (1)
The third digit = permissions for other users in the file's group: read (4), write (2), and execute (1)
The fourth digit = permissions for other users NOT in the file's group: read (4), write (2), and execute (1)

The octal (0-7) value is calculated by adding up the values for each digit
User (rwx) = 4+2+1 = 7
Group(rx) = 4+1 = 5
World (rx) = 4+1 = 5
chmode mode = 0755

Examples

Allow read permission to everyone:
chmod 444 file

Make a file readable and writable by the group and others:
chmod 066 file

Allow everyone to read, write, and execute the file:
chmod 777 file

Symbolic Mode

The format of a symbolic mode is `[ugoa...][[+-=][rwxXs­tugo...]...][,...]'.

Multiple symbolic operations can be given, separated by commas.

A combination of the letters `ugoa' controls which users' access to the file will be changed:

The user who owns it (u)
Other users in the file's group (g)
Other users not in the file's group (o)
All users (a)

If none of these are given, the effect is as if `a' were given, but bits that are set in the umask are not affected.

all users (a) is effectively
user + group + others

The operator '+' causes the permissions selected to be added to the existing permissions of each file; '-' causes them to be removed; and '=' causes them to be the only permissions that the file has.

The letters 'rwxXstugo' select the new permissions for the affected users:

Read (r),
Write (w),
Execute (or access for directories) (x),
Execute only if the file is a directory or already has execute permission for some user (X),
Set user or group ID on execution (s),
Save program text on swap device (t),
The permissions that the user who owns the file currently has for it (u),
The permissions that other users in the file's group have for it (g),
Permissions that other users not in the file's group have for it (o).

Examples
Deny execute permission to everyone:
chmod a-x file

Allow read permission to everyone:
chmod a+r file

Make a file readable and writable by the group and others:
chmod go+rw file

Allow everyone to read, write, and execute the file and turn on the set group-ID:
chmod =rwx,g+s file

Notes:
When chmod is applied to a directory:
read = list files in the directory
write = add new files to the directory
execute = access files in the directory

chmod never changes the permissions of symbolic links. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.

"I think we too often make choices based on the safety of cynicism, and what we're lead to is a life not fully lived. Cynicism is fear, and it's worse than fear - it's active disengagement." - Ken Burns, Historian

Related commands:

chown - Change file owner and group
chflags - Change a file or folder's flags.
install - Copy files and set attributes
ln - Make links between files (hard links, symbolic links)
mount - Mount a file system
umask - Users file creation mask

Equivalent BASH command:

chmod - Change Directory