General Unix

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CHMOD -
Numeric mode:
From one to four octal digitsAny 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)SThe 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
chmod 400 file - Read by owner
chmod 040 file - Read by group
chmod 004 file - Read by world
chmod 200 file - Write by owner
chmod 020 file - Write by group
chmod 002 file - Write by world
chmod 100 file - execute by owner
chmod 010 file - execute by group
chmod 001 file - execute by world
To combine these, just add the numbers together:chmod 444 file - Allow read permission to owner and group and worldchmod 777 file - Allow everyone to read, write, and execute file
Symbolic Mode
The format of a symbolic mode is a combination of the letters +-= rwxXstugoaMultiple symbolic operations can be given, separated by commas. The full syntax is [ugoa...][[+-=][rwxXstugo...]...][,...] but this is explained below. A combination of the letters ugoa controls which users' access to the file will be changed:

• || User || letter ||
• || 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 + othersThe 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:

• || Permission || letter ||
• || 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 ||

ExamplesDeny execute permission to everyone: chmod a-x fileAllow read permission to everyone:chmod a+r fileMake a file readable and writable by the group and others: chmod go+rw file
Make a shell script executable by the user/owner $ chmod u+x myscript.shAllow everyone to read, write, and execute the file and turn on the set group-ID: chmod =rwx,g+s fileNotes:When chmod is applied to a directory: read = list files in the directorywrite = 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.