How do I use a file grep comparison inside a bash if/else statement?

Question

When our server comes up we need to check a file to see how the server is configured.

We want to search for the following string inside our /etc/aws/hosts.conf file:

MYSQL_ROLE=master

Then, we want to test whether that string exists and use an if/else statement to run one of two options depending on whether the string exists or not.

What is the BASH syntax for the if statement?

if [ ????? ]; then
  #do one thing
else
  #do another thing
fi

Answer 1

From grep --help, but also see man grep:

Exit status is 0 if any line was selected, 1 otherwise; if any error occurs and -q was not given, the exit status is 2.

if grep --quiet MYSQL_ROLE=master /etc/aws/hosts.conf; then
  echo exists
else
  echo not found
fi

You may want to use a more specific regex, such as ^MYSQL_ROLE=master$, to avoid that string in comments, names that merely start with "master", etc.

This works because the if takes a command and runs it, and uses the return value of that command to decide how to proceed, with zero meaning true and non-zero meaning false—the same as how other return codes are interpreted by the shell, and the opposite of a language like C.