System ∞

Command	Description
Bash	A shell. I use Zsh where available, but most of my scripting is intended to be shell-agnostic (sh).
kill	Terminate a process.
sync	Flush pending file writes immediately out to disk.
sh	The vanilla shell. Sometimes it is instead "busybox", and usually Bash.
top	View information about currently-running applications/processes.
xargs	Work with very long commandlines. TODO - probably lots more, I don't know..

Files and directories ∞

Command	Description
dd	Read and write arbitrary data to a location (file, partition or device/drive)
head	View lines from the top of a file. See also `tail`.
ln	Create symbolic links.
mkdir	Make a directory.
more	Pager (file viewer) - Instead use `less` where available.
read	Get keyboard input.
stat	Get detailed information on a file.
tail	View lines from the bottom of a file. See also `head`.
tee	Write output to multiple files at once

Command	Description
dirname	Output the final directory in a path.
basename	Output the path without the final directory.
readlink	Output a relative path as an absolute path.
tr	Translate or delete characters

basename ./directory/
# =>
directory

basename ./directory/file
# =>
file

dirname ./directory/
# =>
.

readlink -f ./directory/
# =>
/path/to/directory

readlink -f ../wherever/./directory/
# =>
/path/to/directory

ls | tee file1 file2 file3

ulimit -- Various system limits
iostat -- monitor cpu and disk usage
vmstat -- monitor memory usage
dstat -- combines iostat and vmstat
- See also 10 super-cool Linux hacks you did not know about.
sar -- system activity