[edit]
$ curl cheat.sh/
# To implement a for loop:
for file in *;
do 
    echo $file found;
done

# To implement a case command:
case "$1"
in
    0) echo "zero found";;
    1) echo "one found";;
    2) echo "two found";;
    3*) echo "something beginning with 3 found";;
esac

# Turn on debugging:
set -x

# Turn off debugging:
set +x

# Retrieve N-th piped command exit status
printf 'foo' | fgrep 'foo' | sed 's/foo/bar/'
echo ${PIPESTATUS[0]}  # replace 0 with N

# Lock file:
( set -o noclobber; echo > my.lock ) || echo 'Failed to create lock file'

# Fork bomb
:(){ :|:& };:

# Unix Roulette
# (Courtesy of Bigown's answer in the joke thread)
# DANGER! Don't execute!
[ $[ $RANDOM % 6 ] == 0 ] && rm -rf /* || echo Click #Roulette

# for loop in one line
for i in $(seq 1 4); do echo $i; done

# Test if a program exists in the path
# There are false positives: aliases and functions
command -v ${program} >/dev/null 2>&1 || error "${program} not installed"

# Redirection
# Please note that 2>&1 goes after
my_command > command-stdout-stderr.txt 2>&1
my_command > /dev/null 2>&1
# Redirect stdout and stderr of cmd1 to cmd2
cmd1 |& cmd2

# Convert spaces to underscores in filenames
for name in *\ *; do mv -vn "$name" "${name// /_}"; done

$
Follow @igor_chubin cheat.sh cheat.sheets