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Eavesdrop on your system
This command takes a snapshot of the open files for a PID 1234 then waits 10 seconds and takes another snapshot of the same PID, it then displays the difference between each snapshot to give you an insight into what the application is doing.

Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"

Wait for an already launched program to stop before starting a new command.
Referring to the original post, if you are using $! then that means the process is a child of the current shell, so you can just use `wait $!`. If you are trying to wait for a process created outside of the current shell, then the loop on `kill -0 $PID` is good; although, you can't get the exit status of the process.

Get IPv4 of eth0 for use with scripts
Combines wgzhao's grep | awk | sed into one awk command.

File rotation without rename command
Rotates log files with "gz"-extension in a directory for 7 days and enumerates the number in file name. i.e.: logfile.1.gz > logfile.2.gz I needed this line due to the limitations on AIX Unix systems which do not ship with the rename command.

Fast file backup
This is a BASH feature. The above command will create a backup of "filename" called "filename.DATE", where DATE is the actual day in %Y%m%d (year, month and day numbers together) format.

Create a tar of directory structure only

simulated text generator

Recursively remove directory with many files quickly
rsync'ing an empty directory over a directory to be deleted recursively is much faster than using rm -rf, for various reasons. Relevant only for directories with really a lot of files.

Show apps that use internet connection at the moment. (Multi-Language)
Show apps that use internet connection at the moment. Can be used to discover what programms create internet traffic. Skip the part after awk to get more details, though it will not work showing only unique processes. This version will work with other languages such as Spanish and Portuguese, if the word for "ESTABLISHED" still contain the fragment "STAB"(e.g. "ESTABELECIDO")


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