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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" }

Find all active ip's in a subnet
nmap for windows and other platforms is available on developer's site: http://nmap.org/download.html nmap is robust tool with many options and has various output modes - is the best (imho) tool out there.. from nmap 5.21 man page: -oN/-oX/-oS/-oG : Output scan in normal, XML, s|

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"

Netcat Relay
This is an example of using 3 hosts, in a netcat relay. first host connects to middle host 1 -> 2 Second hosts redirects to target host 1 -> 2 -> 3 I hope this makes sense.

Command to logout all the users in one command
Logs all users out except root. I changed the grep to use a regexp in case a user's username contained the word root.

check open ports (both ipv4 and ipv6)
While `lsof` will work, why not use the tool designed explicitly for this job? (If not run as root, you will only see the names of PID you own)

print shared library dependencies
May be used on (embedded) systems lack ldd

Change proccess affinity.
Set the affinity of a process to a particular core(s). Arguments for processor include a comma separated list, or a range. (example: 1,2 or 0-3) You can use top in smp mode (Press 1) to see the changes to the affinity.

get a rough estimate about how much disk space is used by all the currently installed debian packages
The vaule is expressed in megabytes

Get your default route
This gets you your default route programatically, useful for scripts


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