How can I check if a program exists from a Bash script?

Question

How would I validate that a program exists, in a way that will either return an error and exit, or continue with the script?

It seems like it should be easy, but it\

Answer 1

Late answer but this is what I ended up doing.

I just check if the command I execute returns an error code. If it returns 0 it means program is installed. Moreover you can use this to check the output of a script. Take for instance this script.

foo.sh

#!/bin/bash
echo "hello world"
exit 1 # throw some error code

Examples:

# outputs something bad... and exits
bash foo.sh $? -eq 0 || echo "something bad happened. not installed" ; exit 1

# does NOT outputs nothing nor exits because dotnet is installed on my machine
dotnet --version $? -eq 0 || echo "something bad happened. not installed" ; exit 1

Basically all this is doing is checking the exit code of a command run. the most accepted answer on this question will return true even if the command exit code is not 0.