Searches /var/log/secure for smtp connections then lists these by number of connections made and hosts.

Searches the /var/log/secure log file for Failed and/or invalid user log in attempts.

Here is a command line to run on your server if you think your server is under attack. It prints our a list of open connections to your server and sorts them by amount.

BSD Version:

I find this terribly useful for grepping through a file, looking for just a block of text. There's "grep -A # pattern file.txt" to see a specific number of lines following your pattern, but what if you want to see the whole block? Say, the output of "dmidecode" (as root):

Will show me everything following the battery block up to the next block of text. Again, I find this extremely useful when I want to see whole blocks of text based on a pattern, and I don't care to see the rest of the data in output. This could be used against the '/etc/securetty/user' file on Unix to find the block of a specific user. It could be used against VirtualHosts or Directories on Apache to find specific definitions. The scenarios go on for any text formatted in a block fashion. Very handy.

Some commands (such as netcat) have a port option but how can you know which ports are unused?

The arguments of "seq" indicate the starting value, step size, and the end value of the x-range. "awk" outputs (x, f(x)) pairs and pipes them to "graph", which is part of the "plotutils" package.

Displays six rows and five columns of random numbers between 0 and 1. If you need only one column, you can dispense with the "for" loop.

This example calculates the averages of column one and column two of "file.dat". It can be easily modified if other columns are to be averaged.

Another combination of seq and awk. Not very efficient, but sufficiently quick.

"seq 100" outputs 1,2,..,100, separated by newlines. awk adds them up and displays the sum.

"seq 1 2 11" outputs 1,3,..,11.

Variations:

1+3+...+(2n-1) = n^2

1/2 + 1/4 + ... = 1

This loops through all tables and changes their collations to UTF8. You should backup beforehand though in case some data is lost in the process.

Show the number of failed tries of login per account. If the user does not exist it is marked with *.

Just type 'opened' and get all files currently opened for edit.

This command shows if there are any locked AFS volumes.

The output is a list of AFS volume IDs (or nothing if there are none locked).

Takes a input file (count.txt) that looks like:

1

2

3

4

5

It will add/sum the first column of numbers.

This command converts filenames with embedded spaces in the current directory replacing spaces with the underscore ("_") character.

Can pipe to tail or change the awk for for file size, groups, users, etc.

This command will kill all processes using a directory. It's quick and dirty. One may also use a -9 with kill in case regular kill doesn't work. This is useful if one needs to umount a directory.