Useful Unix Things Everyone Should Know

Recursive Grep

$ find . -name '*.txt' -exec grep 'nslookup' {} \; -print

The -exec option is awesome, but requires the archaic syntax at the end of the statement, which I still don't totally understand. Other commands can be derived easily in this way. The find command works in a left-2-right fashion (ie. -name -> -exec -> -print), if -name returns, then it goes to -exec, and if -exec returns, then it goes to -print, etc. -print is something that always returns true, so putting -print in front of -exec, like:

$ find . -name '*.txt' -print -exec grep 'nslookup' {} \;     (not as useful as the above)

would print all '*.txt' files without regard to the -exec grep command, and the grep would work as you would expect.

Duplex Printing a Postscript file

s/^%%EndProlog/ \<\< \/Duplex true \>\> setpagedevice\ %%EndProlog/g

I don't think that this can be invoked on the command line only b/c the newline will blow up most shells. My suggestion is to save the above link, and make an alias in one of your rc files like:

alias duplexps 'sed -f /home/sbslave/sed/duplexps.sed'

Securing your Home Directory, but Allowing Web Access

The easiest way to check permissions in your home and public_html directory is to try this:

$ cd $ ls -la | head -2 | tail -1      drwxrwx--x 52 cerac001 cix 4096 Jul 28 14:23 ./ $ ls -l public_html      drwxr-x--x 5 cerac001 cix 4096 Jul 28 19:02 public_html/

This permission schema was done with:

$ cd /home $ chmod 771 $USER $ cd $USER $ chmod 751 public_html

Remember:
         0 000 ---
         1 001 --x
         2 010 -w-
         3 011 -wx
         4 100 r--
         5 101 r-x
         6 110 rw-
         7 111 rwx

I'm trusting the people in the cix group to rwx in my home directory, but nobody else. You may wish to block out your own group for added security in case somebody compromises their account. In order to allow WEB access to your ~/public_html/ directory, the execute permssion is the tricky parameter for access to your home directory. Somebody needs execute permission in order to cd into your directory (web browser must perform this operation). Somebody needs read permission to ls your directory (not necessary for the web browser so long as index.html is defined in the ~/public_html directory. Basically, somebody can cd passed your home directory IF they already KNOW something else exists. If you don't have read access, then you would have to previously know that something existed.

A web browser already KNOWS about ~/public_html and already LOOKS for index.html, it ONLY needs execute permission to cd into it. From there, if your anchors in your web page are linked properly, a person can get around. The browser will also need read permission on your *.html files because it needs the html source obviously.

I want my web pages to be accessible by anyone, thus the world permissions must be tweaked. Some systems force the web browser to run as the user nobody who needs world permissions to operate, but it is possible for the browser to run as a group member so if you want added security, you should run your browser in this manner, only make your permissions for the directories within the group context. This is certainly possible, but it may take some tweaking. I, personally, don't do this b/c this information your reading is not corporate sensitive material.

One of the drawbacks of having the public_html directory in your home directory is that you must give your $HOME directory execute permission as described above. This means that everything in that home directory is open to interpretation by individual permissions. Thus, you are then forced to further secure everything above the home directory that is sensitive to ensure that snooping is not allowed. It is also helpful to use the umask and newgrp commands, to ensure that newly created files have restrictions set upon them immediately. Since somebody can cd into the directory, you must make sure that they can not continue doing this up your directory tree. Make sure you understand the distinctions between file and directory permissions. They are very different, but not being aware of the directory permissions is just asking for trouble.

For further reading in this topic, I refer you to the chapter on permissions in "Essential System Administration" by O'Reilly & Associates.

s-lang

What is s-lang?

Why use s-lang instead of curses?

What programs use s-lang?

slrn

mutt

lynx

vim