Fixing "could not open user_jvm_args.txt" on a Forge server

What the file actually is

Modern Forge server installers, from roughly 1.17 onward, generate two things alongside the server jar: a launch script, run.bat on Windows or run.sh on Linux and macOS, and a plain text file called user_jvm_args.txt.

That file holds the JVM flags you are meant to edit, most commonly -Xmx4G for maximum heap. The launch script does not read those flags itself. It hands the file straight to Java using an argfile, a @user_jvm_args.txt argument that tells the JVM "read more arguments from this file". This is standard Java behaviour, not a Forge invention, and nowhere explains it plainly, which is exactly why the error confuses people.

So when the script runs, it is really just:

java @user_jvm_args.txt @libraries/net/minecraftforge/forge/<version>/unix_args.txt "$@"

If that first @ reference fails to resolve, Java refuses to start and prints something about not being able to open or find the file. The message is accurate. It is just rarely about the file being broken.

The three real causes

Wrong working directory. The script assumes user_jvm_args.txt sits next to it, in the same folder. @user_jvm_args.txt is resolved against the shell's current directory, not the script's location. A desktop shortcut, a process manager that launches from a different path, or simply running the script from one directory up all leave the shell somewhere other than the server folder, and the lookup fails even though the file exists a few directories away.

Java missing or not on PATH. If java is not a recognised command, the shell fails before it ever gets to the argfile. The resulting message still reads like a complaint about user_jvm_args.txt, because that is the first argument on the line that failed, but the real problem is that there is no interpreter to run it.

A broken line inside the file. Only lines starting with # are comments. A stray character, a flag split across a line incorrectly, or a # placed after a flag instead of on its own line can break the argfile parser. This is a documented pattern on Forge and AnswerOverflow community threads, and it is easy to introduce by pasting flags from a forum post instead of typing them.

Fix it in order

1. Run the script from inside the server's own folder.

cd /path/to/your/server
./run.sh

Do not launch it from a shortcut or a script elsewhere that calls into this folder. cd into it first, every time.

2. Confirm Java is actually installed and on PATH, in that same terminal.

java -version

If that fails or prints nothing, the launch script was never going to work regardless of what is in user_jvm_args.txt. Install a Java runtime that matches what your Minecraft version needs and make sure it is on PATH before trying again. The Java version checker tells you which major version your Minecraft release requires.

3. Open user_jvm_args.txt in a plain text editor and check it line by line.

Only lines that start with # are comments. Flags go one per line, or space separated on the same line, but a # after a flag on the same line is not a trailing comment, it breaks parsing. A file that looks like this is correct:

# Xmx and Xms set your maximum and starting memory
-Xmx4G
-Xms4G

4. Confirm the file exists at all.

A Forge installer run that was interrupted or run against a corrupted download can skip generating user_jvm_args.txt entirely. If it is genuinely missing, re-run the installer against the same server folder rather than trying to hand-write a replacement.

Editing memory while you are in there

This file is exactly where you set -Xmx, and it is worth getting the number right while you are already fixing the startup error. See how much RAM a Minecraft server needs for sizing it, or use the start command generator to produce a known-good flag set and script for your setup instead of editing this one by hand.