Add Check If The First Argument Is Given as a shell til

shell/check-if-the-first-argument-is-given.md

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+# Check If The First Argument Is Given
+
+In a shell script, you may want to check if an argument was given. Each
+argument is referenced numerically with the `$` prefix, so the first argument
+is `$1`. To check if the first argument is given, you can use the `-z` check.
+
+```bash
+if [ -z "$1" ]
+  then
+    echo "The first argument is missing"
+    exit 1
+fi
+```
+
+The `-z` checks if the argument is a zero-length string (so `""` or undefined
+will be true). If it is missing, then we echo out a message and exit the
+script. This is how I might fashion a script that requires the first argument.
+
+[source](https://stackoverflow.com/questions/6482377/check-existence-of-input-argument-in-a-bash-shell-script)