chmod : Java Glossary

2011-12-04

chmod

Most likely your ISP (Internet Service Provider) uses some Unix variant as its webserver to host your web pages. In Unix there are permissions on each file. Are you may or may not be allowed to look at r=read, change w=write, or x=execute each file. The permissions are separately recorded for myself u=user, other users in my group g=group, and the world at large, including strangers out on the net o=others.

There is a high order bit that tells if a file is actually a directory: d rwx rwx rwx. In that case:

The read bit gives you to right to get a list of files in the directory.
The write bit lets you create new files in that directory, change the attributes of files in that directory or write/delete files in that directory.
The execute bit lets you access files/directories in that directory that you know by name and lets you change into that directory.

The odd-looking chmod 755 name.class command sets the permissions. The permissions are encoded in binary, 111 101 101, then in octal 755, one bit for each permission, 1=ok 0=not ok to do that. 755 means I give myself permission to do everything, but I don’t let others write, though I let them read and execute.

CHMOD Octal Permission Codes
Octal	Meaning
0	---
1	--x
2	-w-
3	-wx
4	r-x
6	rw-
7	rwx

Learn To Count Applet: to sharpen your intuition on how binary and octal work

Windows Attributes

DOS/W3.1/OS2/W95/W98/Me/NT/W2K/XP/W2003/Vista/W2008/W7-32/W7-64 uses different scheme, controlled with the ATTRIB command.

flag	Effect
+A	Set the archive attribute
-A	Clear the archive attribute
+H	Set the hidden attribute
-H	Clear the hidden attribute
+R	Set the read-only attribute
-R	Clear the read-only attribute
+S	Set the system attribute
-S	Clear the system attribute

binary
octal

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