if

Conditionally perform a command.

Syntax
      if test-commands; then
        consequent-commands;
      [elif more-test-commands; then
        more-consequents;]
      [else alternate-consequents;]
      fi

Or in a single line:
     if test-commands; then consequent-commands; fi

The test-commands list is executed, and if it's return status is zero (success), the consequent-commands list is executed.

Although if is most often used with test to return a True/False decision, it can be used with any command that returns an exit code of 0 for success.

If test-commands returns a non-zero status, each elif list is executed in turn, and if its exit status is zero, the corresponding more-consequents is executed and the command completes.

If 'else alternate-consequents' is present, and the final command in the final if or elif clause has a non-zero exit status, the alternate-consequents is executed.

For simple comparisons, a more concise option is to use test along with the conditional operator && instead of IF.

for example
var=snark
[[ "$var" = "snark" ]] && echo "found snark"

or the equivalent using if:

var=snark
if [[ "$var" = "snark" ]]
 then echo "found snark"
fi

Test if the file music.txt exists:

If [[ -e music.txt ]]; then   
  echo "we found the file"
fi

The return status is the exit status of the last command executed, or zero if no condition tested true.

This is a BASH shell builtin, to display your local syntax from the bash prompt type: help if

"Then you admit confirming not denying you ever said that?"
"NO! ... I mean Yes! WHAT?"
I'll put 'maybe.' ~ Bloom County

Related linux commands:

case - Conditionally perform a command.
eval - Evaluate several commands/arguments.
expr - Evaluate expressions.
for - Expand words, and execute commands.
test - Evaluate a conditional expression.
until - Execute commands (until error).
while - Execute commands.
File operators -f
Equivalent Windows command: IF - Conditionally perform a command.