I have a directory containing log files. This command delete all but the 5 latest logs. Here is how it works:

* The ls -t command list all files with the latest ones at the top

* The awk's expression means: for those lines greater than 5, delete.

This will calculate a running standard deviation in one pass and should never have the possibility for overflow that can happen with other implementations. I suppose there is a potential for underflow in the corner case where the deltas are small or the values themselves are small.

This is an on-line algorithm for calculating the mean value for numbers in a column. Also known as "running average" or "moving average".

This command displays a list of lines that are longer than 72 characters. I use this command to identify those lines in my scripts and cut them short the way I like it.

Connect to a machine running ssh using mac address by using the "arp" command

Removes trailing newline; colon becomes record separator and newline becomes field separator, only the first field is ever printed. Replaces empty entries with $PWD. Also prepend relative directories (like ".") with the current directory ($PWD). Can change PWD with env(1) to get tricky in (non-Bourne) scripts.

This is really fast :)

204753

real 0m0.191s

user 0m0.068s

sys 0m0.116s

Checks the Gmail ATOM feed for your account, parses it and outputs a list of unread messages.

For some reason sed gets stuck on OS X, so here's a Perl version for the Mac:

If you want to see the name of the last person, who added a message to the conversation, change the greediness of the operators like this:

Shell timeout variables (TMOUT) can be very liberal about what is classified as 'activity', like having an editor open. This command string will terminate the login shell for an user with more than a day's idle time.

From there, just pkg install the package you need.

Does not require input to function or complete. Number of iterations controlled by shell variable $NUM.

shows also time if its the same year or shows year if installed before actual year and also works if /etc is a link (mac os)

Have wc work on each file then add up the total with awk; get a 43% speed increase on RHEL over using "-exec cat|wc -l" and a 67% increase on my Ubuntu laptop (this is with 10MB of data in 767 files).

needs no GNU tools, as far as I see it

saves one command. Needs GNU grep though :-(

This will drop you into vim to edit all files that contain your grep string.

Show time and date when you installed your OS.

using awk

missed the last char thanks @Josay

This command uses nmap to perform reverse DNS lookups on a subnet. It produces a list of IP addresses with the corresponding PTR record for a given subnet. You can enter the subnet in CDIR notation (i.e. /24 for a Class C)). You could add "--dns-servers x.x.x.x" after the "-sL" if you need the lookups to be performed on a specific DNS server.

On some installations nmap needs sudo I believe. Also I hope awk is standard on most distros.

Only one command and not dependant on full read access to the devices.

simple way to show free swap

This command will print all fields from the given input to the end of each line, starting with the Nth field.