WYNTK: UNIX System Admininistrator

If you can read a file but can’t write it, you can copy the file and make your changes in the copy.

Be careful using the bit syntax for chmod

One day a user complained that she couldn’t cd to one of her directories anymore. It turned out that she wanted to let someone else read several files in her home directory, so she typed “chmod 666 *” in her home directory. She wanted to give everyone permission for all her files.

The problem was that it also changed the permissions on all subdirectories of her home directory.

Directories in UNIX have to be executable in order to cd into them. By running “chmod 666” on a directory, she made it unexecutable.

What she should have done is “chmod a+rw *”, which would have added read/write permissions to all the files and directories without removing the execute bit.

If you need to change a read-only file, what you should do is find the owner of the file and have its permissions changed. But if you can’t find the owner of the file, and your changes really must be done immediately, you can also just copy the file and make your changes in the copy:

    % cp 
 updates ~/updates.mine
    % vi ~/updates.mine
    (edit as needed)

The only caveat is that you need to make sure that your “new” version of the file is copied over the “real” version before more changes are made to it. So send email to the owner:

    % mail mike
    Subject: updates file

    Hey, I needed to make changes to updates, but I didn't have
    write permission, so I made my changes into a copy in my
    home directory named updates.mine. Please stay out of the
    updates file for now. I'll let you know when the changed
    version can be copied into the original.

Otherwise you risk having to merge changes from two different versions of the file.