Create a symbolic link:
ln -s source target
List disk usage of directorys, but limit output to one level. Show size (blocksize) in bytes:
du --max-depth=1 --block-size=1
As above but sort by decending use:
du --max-depth=1 --block-size=1 | sort -nr
Show all printable strings in a binary file:
strings
Monitor network traffic:
netstat -a
Use free command, to find the free memory, or use cat on /proc/meminfo. The following command extracts the free memory and writes to a file called freememory with a timestamp. Useful to add to cron, running every minute so you can check memory retrospectively:
echo `date` - `free | tail -n 3 | head -n 1 | awk '{print $4}'` >> freememory
Show services that run at the various levels:
chkconfig --list
Show current runlevel:
/sbin/runlevel