The chmod Command
Let's look at a summary of chmod's options, and then cover each option in depth:
u user (owner)
o other (world)
a all (user, group, and other)
= set exactly
X conditionally set execute
s Set UID or set GID
t set “sticky” bit
$ chmod a+rwx test_file $ ls -l test_file -rwxrwxrwx 1 eric users
This demonstrates the fourth possible symbol for user when using symbolic mode. We used a to set full permissions for all user classes at once. Let's delete the file and start over in order to demonstrate the difference between the = operator and the + and - operators. (From here on, we'll assume that you know how to get the directory listing, and won't list the ls command.)
$ rm test_file $ touch test_file -rw-rw-r-- 1 eric users $ chmod g+x test_file -rw-rwxr-- 1 eric users
This added execute permission for group.
$ chmod g=x test_file -rw---xr-- 1 eric users
The = operators set group's permissions to execute, and in doing so removed read and write permission. While + and - set or unset the permissions specified, = will set exactly the mode specified and remove any others.
Read, write and execute modes are very straightforward when referring to files. Read and write allow a user to examine and modify/delete data from a file, respectively. Execute allows a user to execute a shell script or binary program. If you ftp a program from one host to another and then try to run it without setting execute permission, it will fail, since ftp does not set execute permission.
For directories, the rules can be a bit more complicated.
Read permission allows a user to examine the contents of a directory.
$ mkdir test_dir $ touch test_dir/foo $ ls test_dir foo $ chmod u-r test_dir $ ls test_dir ls: test_dir: Permission denied
Write permission allows a user to modify the contents of the directory. That means that lack of write permission on a directory does not prevent a user from modifying a file within the directory, if the file's permissions allow it. It does prevent the user from renaming, moving, deleting or creating any file in the directory. This is because a directory is a really a file that contains a list of filenames, and so read and write permission control access to that list.
$ chmod u=rx test_dir dr-xrwxr-x 2 eric users $ touch test_dir/bar touch: test_dir/bar: Permission denied $ mv test_dir/foo ./foo mv: cannot move `test_dir/foo' to `./foo': Permission denied
This property also works the other way. Since write permission allows the modification of directory entries, a user can move or rename a file without permission to examine the contents. This is a very good reason for paying attention to write access for important directories.
$ ls -l test_dir -rw-rw-r-- 2 eric users foo $ chmod u=rwx test_dir $ chmod u=rx test_dir/foo $ cat .bashrc > test_dir/foo bash: test_dir/foo: Permission denied $ mv test_dir/foo ./foo $ ls test_dir (It's empty) $ ls foo foo (It's in our present directory.)
Execute permission for directories (also referred to as search permission) is also very important. Execute permission is necessary for accessing a directory.
$ chmod u=rwx test_dir $ cp ~/.bashrc test_dir (any text file will do) $ chmod u=rw test_dir $ cd test_dir bash: test_dir: Permission denied $ cat test_dir/.bashrc cat: test_dir/.bashrc: Permission denied
This copy of .bashrc does not do us a lot of good. However, setting execute permission for directory and not setting read or write can come in handy.
$ chmod u=x test_dir $ cat test_dir/.bashrc (we see the contents of the file) $ ls test_dir ls: test_dir: Permission denied
A directory that has execute permission only can be used to “hide” files. Only users who know the exact file name and path can access them; this includes both data files and programs.
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
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|The Firebird Project's Firebird Relational Database||Jul 29, 2016|
|Stunnel Security for Oracle||Jul 28, 2016|
|SUSE LLC's SUSE Manager||Jul 21, 2016|
|My +1 Sword of Productivity||Jul 20, 2016|
|Non-Linux FOSS: Caffeine!||Jul 19, 2016|
|Murat Yener and Onur Dundar's Expert Android Studio (Wrox)||Jul 18, 2016|
- The Firebird Project's Firebird Relational Database
- Stunnel Security for Oracle
- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- SUSE LLC's SUSE Manager
- Managing Linux Using Puppet
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Doing for User Space What We Did for Kernel Space
- Google's SwiftShader Released
- SuperTuxKart 0.9.2 Released
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide