Add Check If Command Is Executable Before Using as a unix til

unix/check-if-command-is-executable-before-using.md

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+# Check If Command Is Executable Before Using
+
+When writing a quick bash script, you may want to check if a command exists and
+is executable before trying to call it. This can be done with `command`, a builtin shell command, and the `-v` flag.
+
+> If the -V or -v option is supplied, the exit status is 0 if command was found, and 1 if not.
+
+Knowing that, we can redirect the output of the command to `/dev/null` and then
+short-circuit executing the command if it's not available.
+
+```bash
+command -v pbcopy >/dev/null 2>&1 && echo 'something' | pbcopy
+```
+
+In this example, I execute the `pbcopy` command, which copies text to my system
+clipboard, only if that command is available and executable.
+
+See `man bash` and find the listing for `command` for more details.