bc

Short for “basic calculator”, performs floating point math and various other useful calculations that are not practical with basic shell math support.

expect

Used to work with hard-to-handle command-line tools that require more complex interaction than is possible with a single pipe. For example, you could use an expect script to interact with getty over a tty or other bidirectional connection to log into a remote computer. In general, scripting that requires two-way interaction between the script and a program is most easily done with an expect script.

expr

Evaluates a numerical expression. This command supports basic integer math, and is frequently used for incrementing a loop iterator.

false

Returns a failure exit status (nonzero).

sleep

Pauses execution for a period of time (measured in seconds).

true

Returns a successful exit status (0).