Do comparison without triggering command with the same name - sh

Question

So I'm trying to do a comparison and it just happens to be that the value I'm checking for has the same name as a command on the computer with is a problem.
This is the command i run.
if $current_branch != "HEAD"; then echo '1'; fi;
And here is the result as you can see the HEAD program ran.
400 URL must be absolute
Content-Type: text/plain
Client-Date: Thu, 01 Mar 2018 16:45:48 GMT
Client-Warning: Internal response
200 OK
Cache-Control: no-store, no-cache, must-revalidate, max-age=0
Connection: keep-alive
Date: Thu, 1 Mar 2018 16:45:49 GMT
Server: PWS/8.3.1.0.8
Vary: Accept-Encoding
Content-Type: text/html; charset=utf-8
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Access-Control-Allow-Origin: https://www.head.com
Client-Date: Thu, 01 Mar 2018 16:45:49 GMT
Client-Peer: 151.249.91.221:443
Client-Response-Num: 1
Client-SSL-Cert-Issuer: /C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert SHA2 High Assurance Server CA
Client-SSL-Cert-Subject: /C=US/ST=California/L=Campbell/O=CDNetworks Inc./CN=support2.cdnetworks.net
Client-SSL-Cipher: ECDHE-RSA-AES256-SHA
Client-SSL-Socket-Class: IO::Socket::SSL
Content-Security-Policy: frame-ancestors 'self' https://head.testing-varnish.symmetrics.de
Set-Cookie: geoip=IC; expires=Sat, 31-Mar-2018 16:45:49 GMT; Max-Age=2592000; path=/; domain=head.com
Set-Cookie: lang=en; expires=Sat, 31-Mar-2018 16:45:49 GMT; Max-Age=2592000; path=/; domain=head.com
Set-Cookie: currentLangId=1; expires=Sat, 31-Mar-2018 16:45:49 GMT; Max-Age=2592000; path=/; domain=head.com
Set-Cookie: root=1; expires=Sat, 31-Mar-2018 16:45:49 GMT; Max-Age=2592000; path=/; domain=head.com
X-Frame-Options: SAMEORIGIN
X-Px: nc h0-s42.p1-arn ( h0-s4012.p6-lhr>CONN), nc h0-s4012.p6-lhr ( h0-s4102.p11-fra), nc h0-s4102.p11-fra ( origin)
So how do i avoid this from happening and not have command collide with the string value i want to compare?

Answer 1

The argument to if is always a command. You are probably looking for the special command [ (aka test) which compares strings.
if [ "$current_branch" != "HEAD" ]; then
echo '1'
fi
Take note of the requirement to have spaces on both sides of the [ and ] tokens, and the requirement to quote strings which might contain shell metacharacters.
Depending on whether you require pre-POSIX compatibility, you might also want to invert the negation. On the other hand, in modern shells like Bash and Ksh, you probably want to use the replacement [[ which is somewhat more robust and versatile.
The requirement to have a terminating ] for symmetry is purely aesthetic, but still a requirement. It's pretty quirky that the original Bourne shell didn't have string comparison features built into the shell itself, but, well, this is what we ended up with.
As an aside, if your example is representative of your actual code, you might want to use the somewhat more parsimonious shorthand syntax
[ "$current_branch" = "HEAD" ] || echo '1'