Shell Reference

cat displays the contents of its inputs.
cd path changes the current working directory.
cp old new copies a file.
find finds files with specific properties that match patterns.
grep selects lines in files that match patterns.
head displays the first few lines of its input.
ls path prints a listing of a specific file or directory; ls on its own lists the current working directory.
man command displays the manual page for a given command.
mkdir path creates a new directory.
mv old new moves (renames) a file or directory.
pwd prints the user's current working directory.
rm path removes (deletes) a file.
rmdir path removes (deletes) an empty directory.
sort sorts its inputs.
tail displays the last few lines of its input.
touch path creates an empty file if it doesn't already exist.
wc counts lines, words, and characters in its inputs.
whoami shows the user's current identity.

/path/from/root is an absolute path.
/ on its own refers to the root of the filesystem.
path/without/leading/slash is a relative path.
. refers to the current directory, .. to its parent.
* matches zero or more characters in a filename, so *.txt matches all files ending in .txt.
? matches any single character in a filename, so ?.txt matches a.txt but not any.txt.

command > file redirects a command's output to a file.
first | second connects the output of the first command to the input of the second.

A for loop repeats commands once for every thing in a list:

for variable in name_1 name_2 name_3
do
    ...commands refering to $variable...
done

Use $name to expand a variable (i.e., get its value).
history displays recent commands, and !number to repeat a command by number.
bash filename runs commands saved in filename.
$* refers to all of a shell script's command-line parameters.
$1, $2, etc., refer to specified command-line parameters.
$(command) inserts a command's output in place.