Linux – Detect in C if outputting to a terminal

The simplest and most widely available method to get user input at a shell prompt is the read command. The best way to illustrate its use is a simple demonstration:

while true; do
    read -p "Do you wish to install this program?" yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

Another method, pointed out by Steven Huwig, is Bash's select command. Here is the same example using select:

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
    case $yn in
        Yes ) make install; break;;
        No ) exit;;
    esac
done

With select you don't need to sanitize the input – it displays the available choices, and you type a number corresponding to your choice. It also loops automatically, so there's no need for a while true loop to retry if they give invalid input.

Also, Léa Gris demonstrated a way to make the request language agnostic in her answer. Adapting my first example to better serve multiple languages might look like this:

set -- $(locale LC_MESSAGES)
yesptrn="$1"; noptrn="$2"; yesword="$3"; noword="$4"

while true; do
    read -p "Install (${yesword} / ${noword})? " yn
    if [[ "$yn" =~ $yesexpr ]]; then make install; exit; fi
    if [[ "$yn" =~ $noexpr ]]; then exit; fi
    echo "Answer ${yesword} / ${noword}."
done

Obviously other communication strings remain untranslated here (Install, Answer) which would need to be addressed in a more fully completed translation, but even a partial translation would be helpful in many cases.

Finally, please check out the excellent answer by F. Hauri.

Use the built-in time keyword:

$ help time

time: time [-p] PIPELINE
    Execute PIPELINE and print a summary of the real time, user CPU time,
    and system CPU time spent executing PIPELINE when it terminates.
    The return status is the return status of PIPELINE.  The `-p' option
    prints the timing summary in a slightly different format.  This uses
    the value of the TIMEFORMAT variable as the output format.

Example:

$ time sleep 2

real    0m2.009s
user    0m0.000s
sys     0m0.004s