How do you use if, with an example?

if exist notas.txt echo existe — Prints existe only if the file is in the folder.

if

if [not] condición orden [else orden]

Windows CMD · Terminal · reference only

Runs a command only if a condition is met: if a file exists, if two texts are equal, if the error code is greater than a value...

if [not] condición orden [else orden]

exist fichero
True if the file or folder exists.
errorlevel N
True if the last command's exit code is N or greater.
defined variable
True if the environment variable is defined.
a==b
True if the two texts are equal; they are usually put in quotes.
/i
Ignores case differences in text comparisons.
not
Inverts the condition.
EQU NEQ LSS LEQ GTR GEQ
Numeric operators: equal, not equal, less, less or equal, greater and greater or equal.
else
Command that runs if the condition is not met.

if exist notas.txt echo existe
Prints existe only if the file is in the folder.
if not exist backup mkdir backup
Creates the backup folder only if it does not exist.
if /i "%OPCION%"=="s" (echo Sí) else (echo No)
Compares ignoring case and chooses between two answers.
if errorlevel 1 echo La orden anterior falló
Detects an error from the previous command.

else must be on the same line as the closing parenthesis of the previous command.
Put variables in quotes so an empty value does not break the comparison.