See Tutorial on chmod on How to Geek site.

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Usage

Note that the flags (options) must come before position parameters.

$ man chmod

Linux:

chmod - change file mode bits  

chmod [OPTION]... MODE[,MODE]... FILE...
chmod [OPTION]... OCTAL-MODE FILE...
chmod [OPTION]... --reference=RFILE FILE...

macOS:

NAME
     chmod -- change file modes or Access Control Lists

SYNOPSIS
     chmod [-fv] [-R [-H | -L | -P]] mode file ...
     chmod [-fv] [-R [-H | -L | -P]] [-a | +a | =a] ACE file ...
     chmod [-fhv] [-R [-H | -L | -P]] [-E] file ...
     chmod [-fhv] [-R [-H | -L | -P]] [-C] file ...
     chmod [-fhv] [-R [-H | -L | -P]] [-N] file ...

Mode

You can give a mode like this.

The default is all users.

$ # Add using plus.
$ chmod +x PATH 
$ chmod +xw PATH 
$ # Remove using minus.
$ chmod -x PATH

Add for owner only.

$ chmod u+x PATH

Add for other only.

$ chmod o+x PATH

Add for group only.

$ chmod g+x PATH

Remove read access for other and group.

$ chmod og-r PATH

Octal mode

Use an octal mask which will overwrite all 3 octets.

$ chmod 644 PATH
$ chmod 755 PATH

Flags

Note that flags are different on Linux and macOS. Only Linux supports the longer form of a flag.

  • -v, --verbose - Output a diagnostic for every file processed. Useful for recursive use.
  • -c - Like verbose, but only show if a file was changed.
  • -R, --recursive - Change files and directories recursively

Manpage

Meanings

Target

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

Control Description
u the user who owns it
g other users in the file’s group
o other users not in the file’s group
a all users

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.

Access types

These appear for permissions when using ls -l.

e.g.

-rw-------
lrwxr-xr-x
drwxr-xr-x
Symbol Description
r Read.
w Write.
x Execute.

Other symbols:

Symbol Description
d Directory - appears for directories.
l Symbolic link.
t Sticky
s The set-user-ID-on-execution and set-group-ID-on-execution bits.
+ Access control.

Access control is visible with ls -le

e.g.

drwxr-xr-x+

Bits

rwx

Binary:

101

Number:

= 4*1 + 2*0 + 1*1
= 5

Octal numbers for a single level

Increasing levels of the octal number and what is means.

Permissions Binary number Octal number Description
r-- 100 4 Read only.
r-x 101 5 Read and execute.
rw- 110 6 Read and write.
rwx 111 7 Read, write and execute.
  • The above can be applied to any of the levels - u, g and o.
  • The octal number ranges from 0 to 7 but still uses base 10 so just think of it as plain number with a limited range.

Permissions across levels

Some common use of 3 octal values and meanings. Using three octal numbers.

Permissions Description
444 Read only for everyone.
544 Only owner can write.
644 Only owner can execute (but not wite)
766 Writable for everyone. But only owner can execute.
755 Executable for everyone. But only owner can write.

Examples

Executable file

Add permissions, for all users.

$ chmod +x PATH

Remove permissions:

$ chmod -x PATH

Protect visiblity

Prevent group from reading.

$ chmod g-r PATH

Prevent all from reading.

$ chmod -r PATH

Make only current user able to read and write, others and groups can do nothing.

$ chmod 600 PATH

Allow non-owners to write to directory

Give “other” users who are not owners access to write to a directory.

$ chmod o+w PATH

Restore

Reset to the permissions set when making a new item.

Reset file

$ chmod 544 PATH

Resulting bits:

-rw-r--r--

Reset directory

Like file, except executable bit for all levels.

$ chmod 755 directory

Resulting bits:

drwxr-xr-x

Reset all levels

Note if you also want to update the contents of a directory, you need to use the recursive flag. But be careful to not run this against all files in the directory.

Recommended - use the find command against files and then against directories.