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Linux chmod file permission tips

Linux Tips by Burleson Consulting

This is an excerpt from "Easy Linux Commands" by Linux guru Jon Emmons.  You can purchase it for only $19.95 (30%-off) at this link.

The read, write and execute permissions apply slightly differently to directories than they do to files. The read permission on a directory controls the ability to list the contents of that directory. In this example we?ll create a directory and place a blank file in it. We?ll then modify the permissions on the directory so the owner cannot see the contents.

$ mkdir secret_dir
$ touch secret_dir/my_secret.txt
$ ls secret_dir/
$ chmod u-r secret_dir/
$ ls secret_dir/
ls: secret_dir/: Permission denied
$ cd secret_dir/
$ ls
ls: .: Permission denied
$ cd ../

We see that we get a Permission denied error when trying to view the contents of the directory when the read permission has been revoked. Despite not being able to see what is in the directory we can still change our working directory to that directory.

The write permission on a directory behaves somewhat as expected. If a user has write on a directory they can create or remove files from that directory even if they are not the owner of the files. This is important to note as giving a user, group or other users write on a directory with other user?s files in it will allow them to delete other users files.

Now we?ll give read permissions back to the owner and revoke the execute permission:

$ chmod u+r secret_dir/
$ chmod u-x secret_dir/
$ ls secret_dir/
$ cd secret_dir/
-bash: cd: secret_dir/: Permission denied

We can now view the contents of the directory again but look at what happened when we tried to cd into it! Not having the execute permission on a directory will prevent you from changing into that directory even though you can view the contents. It is understandable how this can cause some confusion.



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