I don't understand this command

[Socrates440]

I am reading a textbook about how to use linux and I am in the chapter that introduces shell scripting. Here is the link if you would like to see the book/chapter that I am referring to (http://rute.2038bug.com/node10.html.gz).

Anyways, the book told me to create a file with the name <filename>.sh which I did. Next it asked me to use the command chmod 0755 myfile.sh. I have googled chmod and I have searched around and I don't understand what this command does. It is my understanding that Chmod changes permissions? Is that right? What will this command do?

Thanks!

[jpollard]

It changes the file permissions.

When a file is created by default, it will get "-rw-rw-r--" (0334). This means the file is not executable... and only readable by world.

Group and owner can read and write (permitting users in the group to modify the file).

"chmod 0755 ..." changes the permissions to "rwxr-xr-x" (owner can read/write/execute, group can read/execute, and world can read/execute). The change of group write means that other members of the group cannot modify an executable, and introduce a security issue by getting something they want run to be run by you.

[Socrates440]

Thanks for the info!

What group are you refering to? Also, where are you getting the permission equivalents for those numbers? Is there a table that I can look up?

[jpollard]

The group is whatever group is associated with the file. The default in Fedora is that the default group of the user is also assigned to the user... which makes both group and user equivalent. Some systems (or those modified by the admin) may be given a default group that has multiple users. The group can be shown by "ls -l", which will show the symbolic representation of the owner/group and permissions (as well as the size of the file).

You can find some tutorial information at http://www.tutorialspoint.com/unix/u...permission.htm
on the symbolic/numeric representations.

The information is actually in the chmod man page, but not in an easily read format. It used to be there in a table, but I think that was back with the BSD man pages.. and has been rewritten since...

For some information on all the access bits (and there are more) you can look at the "man 2 stat" (the man page for the stat system call) which goes into detail on each bit, though it doesn't go into the symbolic representation as used by the ls command.

[Socrates440]

I see where the permissions for user, group, and world (Who is world by the way? Doesn't everybody who is logged on have to be logged on as some form of user?). They come from the 755 right? 7=all permissions, 5 = rx 5=rx. What is the zero for though, and why are there four digits instead of 3?

Also, in the tutorial page you sent me, it gives the example:
amrood]$chmod o+wx testfile
[amrood]$ls -l testfile
-rwxrwxrwx 1 amrood users 1024 Nov 2 00:10 testfile

Who is o?

[george_toolan]

Those numbers are octal. Wikipedia has a nice explanation including the missing table. See http://en.wikipedia.org/wiki/Filesystem_permissions

o is short of "others" read other users who a logged into the system, but are not the owner (u like "user") of the file and not a member of the "group" (g). See

man chmod

[jpollard]

BTW, having an executable file rwxrwxrwx is a way to have a virus - the executable (script or binary) can be replaced by anyone... So you no longer have any control over what the program/script does.