[edit]
$ curl cheat.sh/
# To implement a for loop:
for WORD in LIST
do
    COMMANDS
done
# For example:
for CurDay in Monday Tuesday Wednesday Thursday Friday Saturday Sunday
do
    printf "%s\n" "$CurDay"
done

# To implement a case statement:
case $1 in
    0)
        echo "Found a '0'." ;;
    1)
        echo "Found a '1'." ;;
    2)
        echo "Found a '2'." ;;
    3*)
        echo "Something beginning with '3' found." ;;
    '')
        echo "Nothing (null) found." ;;
    *)
        echo "Anything else found." ;;
esac

# Turn on built-in Bash debugging output:
set -x
# Turn the above off again:
set +x

# Retrieve N-th piped command exit status
printf 'foo' | grep -F '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. Do not run this! Has the potential to wreak havoc. It repeatedly
# and quickly spawns a bazillion processes until the system eventually locks up.
:(){ :|:& };:
# An alternative, easier-to-understand version without the obfuscation:
func(){ func | func & }; func

# Unix Roulette, courtesy of Bigown's answer in the joke thread.
#
#   DANGER! Don't execute!
#
# Luckily, most modern setups have `--preserve-root` on by default, so this will be
# null and void, but even so, not even remotely worth the risk!
[ $[ $RANDOM % 6 ] == 0 ] && rm -rf /* || echo Click #Roulette

# A for loop one-liner.
for CurIter in {1..4}; do echo "$CurIter"; done
# Alternative, slightly-cleaner syntax:
for CurIter in {1..4}; { echo "$CurIter"; }

# Test for a variable being equal to (`-eq`) an integer (`0`).
if [ $var -eq 0 ]; then
    printf "Variable '\$var' is equal to '0'.\n"
fi

# Test for a `PATH` executable existing as a file, but note that aliases and
# functions will also output and result in a `0` exit status.
command -v ${program} >/dev/null 2>&1 || error "${program} not installed"
# However, that is a solution commonly found in a script using the Bourne shell, so
# in this case, an alternative, Bash-like, and more accurate version could be:
if ! type -fP bash > /dev/null 2>&1; then
    printf "ERROR: Dependency 'bash' not met." >&2
    exit 1
fi

# Send both STDOUT and STDERR from COMMAND to FILE. The `2>&1` must go at the end.
COMMAND > FILE 2>&1
# Send STDOUT and STDERR from COMMAND to `/dev/null`, where data goes to die.
COMMAND > /dev/null 2>&1
# Pipe the STDOUT and STDERR from COMMAND_1 to COMMAND_2.
COMMAND_1 |& COMMAND_2

# Verbosely convert whitespaces (` `) to underscores (`_`) in the names of files.
for name in *\ *; do mv -vn "$name" "${name// /_}"; done

$
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