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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"

Show a 4-way scrollable process tree with full details.
If you want a visual representation of the parent/child relationships between processes, this is one easy way to do it. It's useful in debugging collections of shell scripts, because it provides something like a call traceback. When a shell script breaks, just remember "awwfux".

Show directories
Show only the subdirectories in the current directory. In the example above, /lib has 135 files and directories. With this command, the 9 dirs jump out.

Find the process you are looking for minus the grepped one
faster ;) but your idea is really cool

Redirect incoming traffic to SSH, from a port of your choosing
Stuck behind a restrictive firewall at work, but really jonesing to putty home to your linux box for some colossal cave? Goodness knows I was...but the firewall at work blocked all outbound connections except for ports 80 and 443. (Those were wide open for outbound connections.) So now I putty over port 443 and have my linux box redirect it to port 22 (the SSH port) before it routes it internally. So, my specific command would be: $iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-ports 22 Note that I use -A to append this command to the end of the chain. You could replace that with -I to insert it at the beginning (or at a specific rulenum). My linux box is running slackware, with a kernel from circa 2001. Hopefully the mechanics of iptables haven't changed since then. The command is untested under any other distros or less outdated kernels. Of course, the command should be easy enough to adapt to whatever service on your linux box you're trying to reach by changing the numbers (and possibly changing tcp to udp, or whatever). Between putty and psftp, however, I'm good to go for hours of time-killing.

Delete all but the latest 5 files, ignoring directories

let a cow tell you your fortune

Convert seconds to [DD:][HH:]MM:SS
Converts any number of seconds into days, hours, minutes and seconds. sec2dhms() { declare -i SS="$1" D=$(( SS / 86400 )) H=$(( SS % 86400 / 3600 )) M=$(( SS % 3600 / 60 )) S=$(( SS % 60 )) [ "$D" -gt 0 ] && echo -n "${D}:" [ "$H" -gt 0 ] && printf "%02g:" "$H" printf "%02g:%02g\n" "$M" "$S" }

Rename files in batch

determine if tcp port is open
for udp nmap -sU -p 80 hostname


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