contrib: add check for wget command in install_db4.sh #23658

This PR is motivated by https://github.com/bitcoin/bitcoin/commit/7bb8eb0bc352b47ee962283898f9becbb4f36c62 commit (see also #23579) and ensures that install_db4.sh will check for curl and wget utilities. Currently, the conditional statement in the http_get() function assumes that wget is always available but we actually do not know it since there is no check or validation for the wget command. So let's make sure that we check for both commands and print an error message if they are missing.

ACK b062da009001c1beb362169d700663d7220eef5e

It makes sense not to assume by default that wget is preinstalled on an OS. I found that curl and wget are not installed by default on Windows OS.

In that case, running this file would cause a ‘wget not found’ error. Though a person might understand the error and solve it accordingly, it doesn’t hurt to make it more verbose and adequately explain that ‘both curl and wget are not available. Please install one.’

Tested that the change works correctly by using the following patch:

--- a/contrib/install_db4.sh
+++ b/contrib/install_db4.sh
@@ -53,9 +53,9 @@ http_get() {
   #
   if [ -f "${2}" ]; then
     echo "File ${2} already exists; not downloading again"
-  elif check_exists curl; then
+  elif ! check_exists curl; then
     curl --insecure --retry 5 "${1}" -o "${2}"
-  elif check_exists wget; then
+  elif ! check_exists wget; then
     wget --no-check-certificate "${1}" -O "${2}"
   else
     echo "Simple transfer utilities 'curl' and 'wget' not found. Please install one of them and try again."

I got the following output with this patch:

Screenshot:

ACK https://github.com/bitcoin/bitcoin/pull/23658/commits/b062da009001c1beb362169d700663d7220eef5e

Thanks for your contribution! Tested ACK b062da009001c1beb362169d700663d7220eef5e

$ mkdir /tmp/bin
$ ln -s $(which patch mkdir) /tmp/bin
$ bash
$ export PATH="/tmp/bin"
$ contrib/install_db4.sh "$PWD"
Simple transfer utilities 'curl' and 'wget' not found. Please install one of them and try again.
$ exit