Update to the latest Fish shell version

Oct 13, 2025 by wbrum.org 145 views

Introduction

As a user of the Fish shell, you may have encountered a thread panic when pressing Shift+p in Normal mode. This issue can be frustrating, especially when you're in the middle of a task. In this article, we'll delve into the details of this problem and explore possible solutions.

Understanding the Issue

When you press Shift+p in Normal mode, it triggers a thread panic in the Fish shell. The panic message indicates that there's an attempt to subtract with overflow, which is a common issue in programming. The message also suggests that you run with the RUST_BACKTRACE=1 environment variable to display a backtrace.

Analyzing the Backtrace

To better understand the issue, let's analyze the backtrace provided by the Fish shell. The backtrace shows a series of function calls that lead to the panic. Here's a breakdown of the key functions involved:

__pthread_kill_implementation: This function is responsible for killing a thread. In this case, it's called with a signal number of 6, which indicates a segmentation fault.
__pthread_kill_internal: This function is a wrapper around __pthread_kill_implementation. It's called with the same signal number and thread ID.
__GI_raise: This function is responsible for raising an exception. In this case, it's called with a signal number of 6.
std::sys::pal::unix::abort_internal: This function is responsible for aborting the program. It's called from __GI_raise.
std::process::abort: This function is responsible for aborting the process. It's called from std::sys::pal::unix::abort_internal.
fish::panic::panic_handler: This function is responsible for handling panics in the Fish shell. It's called from std::process::abort.
std::panicking::rust_panic_with_hook: This function is responsible for handling panics in Rust. It's called from fish::panic::panic_handler.
std::panicking::begin_panic_handler: This function is responsible for beginning the panic handler. It's called from std::panicking::rust_panic_with_hook.
std::sys::backtrace::__rust_end_short_backtrace: This function is responsible for ending the short backtrace. It's called from std::panicking::begin_panic_handler.
rust_begin_unwind: This function is responsible for beginning unwinding. It's called from std::sys::backtrace::__rust_end_short_backtrace.
core::panicking::panic_fmt: This function is responsible for formatting the panic message. It's called from rust_begin_unwind.
core::panicking::panic_const_sub_overflow: This function is responsible for handling panics caused by subtracting with overflow. It's called from core::panicking::panic_fmt.
fish::reader::Reader::handle_readline_command: This function is responsible for handling readline commands in the Fish shell. It's called from core::panicking::panic_const_sub_overflow.
fish::reader::Reader::handle_char_event: This function is responsible for handling character events in the Fish shell. It's called from fish::reader::Reader::handle_readline_command.
fish::reader::Reader::readline: This function is responsible for reading a line in the Fish shell. It's called from fish::reader::Reader::handle_char_event.
fish::reader::read_i: This function is responsible for reading an integer in the Fish shell. It's called from fish::reader::Reader::readline.
fish::reader::reader_read: This function is responsible for reading from a file descriptor in the Fish shell. It's called from fish::reader::read_i.
fish::throwing_main: This function is responsible for the main entry point of the Fish shell. It's called from fish::reader::reader_read.
core::ops::function::FnOnce::call_once: This function is responsible for calling a function once. It's called from fish::throwing_main.
fish::panic::panic_handler: This function is responsible for handling panics in the Fish shell. It's called from core::ops::function::FnOnce::call_once.
fish::main: This function is responsible for the main entry point of the Fish shell. It's called from fish::panic::panic_handler.

Identifying the Cause

Based on the backtrace, it appears that the issue is caused by a subtract with overflow in the fish::reader::Reader::handle_readline_command function. Specifically, the line 2941:76 in the src/reader.rs file contains a - 1 that's causing the overflow.

Possible Solutions

To fix this issue, you can try the following solutions:

Update to the latest Fish shell version: Make sure you're running the latest version of the Fish shell. You can update to the latest version by running git pull in the Fish shell repository.
Place "exit 0" in the beginning of your config: Try placing exit 0 in the beginning of your Fish shell configuration file. This may help prevent the thread panic.
Run with RUST_BACKTRACE=1: Run the Fish shell with the RUST_BACKTRACE=1 environment variable set. This will display a backtrace when the thread panic occurs.
Use a debugger: Use a debugger like GDB to debug the Fish shell. This will allow you to step through the code and identify the exact location of the issue.

Conclusion

Q: What is a thread panic in the Fish shell?

A: A thread panic in the Fish shell is an error that occurs when the shell encounters an unexpected situation, such as a segmentation fault or an attempt to subtract with overflow. This can cause the shell to crash and display a panic message.

Q: What is the cause of the thread panic when pressing Shift+p in Normal mode?

A: The cause of the thread panic when pressing Shift+p in Normal mode is a subtract with overflow in the fish::reader::Reader::handle_readline_command function. Specifically, the line 2941:76 in the src/reader.rs file contains a - 1 that's causing the overflow.

Q: How can I fix the thread panic when pressing Shift+p in Normal mode?

A: To fix the thread panic when pressing Shift+p in Normal mode, you can try the following solutions:

Update to the latest Fish shell version: Make sure you're running the latest version of the Fish shell. You can update to the latest version by running git pull in the Fish shell repository.
Place "exit 0" in the beginning of your config: Try placing exit 0 in the beginning of your Fish shell configuration file. This may help prevent the thread panic.
Run with RUST_BACKTRACE=1: Run the Fish shell with the RUST_BACKTRACE=1 environment variable set. This will display a backtrace when the thread panic occurs.
Use a debugger: Use a debugger like GDB to debug the Fish shell. This will allow you to step through the code and identify the exact location of the issue.

Q: What is the significance of the backtrace in the thread panic?

A: The backtrace in the thread panic is a list of function calls that led to the panic. It can help you identify the exact location of the issue and understand the sequence of events that led to the panic.

Q: How can I use the backtrace to debug the thread panic?

A: To use the backtrace to debug the thread panic, you can follow these steps:

Run the Fish shell with the RUST_BACKTRACE=1 environment variable set.
Press Shift+p in Normal mode to trigger the thread panic.
Use a debugger like GDB to examine the backtrace and identify the exact location of the issue.
Use the debugger to step through the code and understand the sequence of events that led to the panic.
Modify the code to fix the issue and prevent the thread panic from occurring.

Q: What are some best practices for debugging thread panics in the Fish shell?

A: Some best practices for debugging thread panics in the Fish shell include:

Use a debugger: Use a debugger like GDB to debug the Fish shell and examine the backtrace.
Run with RUST_BACKTRACE=1: Run the Fish shell with the RUST_BACKTRACE=1 environment variable set to display a backtrace when the thread panic occurs.
Update to the latest Fish shell version: Make sure you're running the latest version of the Fish shell to ensure that you have the latest bug fixes and features.
Place "exit 0" in the beginning of your config: Try placing exit 0 in the beginning of your Fish shell configuration file to prevent the thread panic from occurring.

Q: How can I prevent thread panics from occurring in the Fish shell?

A: To prevent thread panics from occurring in the Fish shell, you can follow these best practices:

Use a debugger: Use a debugger like GDB to debug the Fish shell and examine the backtrace.
Run with RUST_BACKTRACE=1: Run the Fish shell with the RUST_BACKTRACE=1 environment variable set to display a backtrace when the thread panic occurs.
Update to the latest Fish shell version: Make sure you're running the latest version of the Fish shell to ensure that you have the latest bug fixes and features.
Place "exit 0" in the beginning of your config: Try placing exit 0 in the beginning of your Fish shell configuration file to prevent the thread panic from occurring.

Conclusion

In conclusion, the Shift+p in Normal mode causes a thread panic in the Fish shell due to a subtract with overflow in the fish::reader::Reader::handle_readline_command function. To fix this issue, you can try updating to the latest Fish shell version, placing exit 0 in the beginning of your config, running with RUST_BACKTRACE=1, or using a debugger. By following these best practices, you can prevent thread panics from occurring in the Fish shell and ensure a smooth and stable user experience.