there are some special automatically set environment variabels that might be interesting to know…

echo $$; # PID of currently running shell
echo $?; # return error code of last run program (before echo)
echo $0; # return /path/and/way the script/command was started (by a softlink or directly?)
echo $#; # number of parameters passed to script/program

Links:

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_02.html

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