add a constant string to the end of the terminal command filename

You can also use find to find and rename the appropriate file.

find.!-name*_X.jpg-type f-execdir sh-c'mv "$1" "${1%.jpg}_X.jpg"'_{}\;

How about this shellscript?

#!/bin/sh

while [-n "$1" ]
do
    fullname = $1
    extension=${fullname##*.}
    filename = ${fullname%.*}
    suffix = ${filename##*_}
    if ["$suffix"!="X"]; then
        mv${filename}"."${extension}${filename}"_X."${extension}
    fi
    shift
done

The answer from @metropolis in the comment may be sufficient, but
Save the above to a file, try saving it to the same directory as the jpeg file, execute the command chmod+x755 filename, and then try ./filename*.jpg.You will get the result of your expectations.

Once the execution is confirmed, from Terminal, click

 mkdir-p/usr/local/bin
sudo cp filename /usr/local/bin

Run two commands to move the command to an executable location that does not pollute the system.
Also, if your shell is bash (this shell is usually used)

PATH=/usr/local/bin:$PATH

Look for the line and add it if you don't have one.
Now, the next time you open Terminal, you can execute the command to add _x from any directory if the filename *.jpg does not have _X at the end of the jpeg file.

shift from while [-n "$1"] to done on the previous line. It handles arguments sequentially and can be applied, so there is no loss in remembering them.