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

Check for multiple dependencies and inform status to end users

for cmd in latex pandoc; do
  printf '%-10s' "$cmd"
  if hash "$cmd" 2>/dev/null; then
    echo OK
  else
    echo missing
  fi
done

Sample output:

latex     OK
pandoc    missing

Adjust the 10 to the maximum command length. It is not automatic, because I don't see a non-verbose POSIX way to do it: How can I align the columns of a space separated table in Bash?

Check if some apt packages are installed with dpkg -s and install them otherwise.

See: Check if an apt-get package is installed and then install it if it's not on Linux

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