CHMOD
chmod stands for ‘change mode’. This allows us to change access permissions for files.
Inside your home directory on Tesla, make a directory called practiceChmod.
(Don’t do this on any directories already made.)
Inside, make a .txt file that just says “My name is [insert name here].” Call the file name.txt.
After exiting VIM (or your text editor of choice), stay in the practiceChmod directory. If you type ls -l, that is a long form of listing the contents of the directory. You’ll see a string listed first that’s 10 characters long. This string tells us the permissions of the file or directory (and if it’s a file or directory).
For example:
-rwx-wx-wx
The first character is either d or -. If the first character is a d, then that item is a directory. If it’s a dash, it’s a file.
Think of everything after the first character as three sets of three (for a total of 9 chars). You will either see an ‘r’, ‘w’, ‘x’, or ‘-’.
- The ‘
r’ stands for read - The ‘
w’ stands for write - The ‘
x’ stands for execute
___ ___ ___ ___ ___ ___ ___ ___ ___ ___
d r w x
or or or or
- - - -
|____USER______| |___GROUP______| |____WORLD_____|
Let’s just think about the last 9 digits from now on (this is what we change with chmod). If we have rwx------ as that string, then the user has read, write, and execute privileges but no one else does.
Each one of these is a bit, either on (letter shown) or off (dash shown). That example (last 9 digits only), would be 111 000 000. If we label r = 4, w = 2, and x = 1 (notice - in sets of three, these are the fours place, twos place, and ones place), then having rwx all set would be 4 + 2 + 1 = 7.
Here are all the options:
- 0 = no permissions whatsoever; this person cannot read, write, or execute the file
- 1 = execute only
- 2 = write only
- 3 = write and execute (1+2)
- 4 = read only
- 5 = read and execute (4+1)
- 6 = read and write (4+2)
- 7 = read and write and execute (4+2+1)
Questions
This are questions that could be asked for quiz, test, or an exam.
What would running ‘chmod 755 me.txt’ do to the file?
User: All
Group: Read and Execute
World: Read and Execute
What about ‘chmod 622 me.txt’?
User: Read and Write
Group: Write
World: Write
What about ‘chmod 000 me.txt’?
User: No Access
Group: No Access
World: No Access