Why does "stty: stdin isn't a terminal" error occur on macOS in iTerm?

The error message "stty: stdin isn't a terminal" on macOS in iTerm, or any terminal emulator, typically indicates that the standard input (stdin) of the stty command is not connected to a valid terminal device.

Here’s a breakdown of why this happens and how to address it:

1. Understanding the Error:

- The stty command (short for "set tty") is used to configure the settings of a terminal device. It expects to be run in a context where stdin is connected to an actual terminal. When stdin is redirected from a file, a pipe, or another non-terminal source, stty cannot operate and throws this error.

2. Common Causes:

- Piping or Redirection: The most common cause is running stty in a context where stdin is being redirected. For instance:

cat somefile.txt | stty -a

In this case, the output of cat is being piped to stty, which expects a terminal, not the content of somefile.txt.

- Background Processes: Running a command that includes stty in the background or through a script executed in a non-interactive shell can lead to this issue. Background processes might not have a proper terminal attached to their stdin.

- Cron Jobs: Cron jobs often run in a non-interactive environment. If your cron job involves stty, it will likely fail.

- SSH and Remote Execution: When executing commands remotely via SSH, especially non-interactive commands, stty might not find a terminal.

3. Solutions and Workarounds:

- Avoid Redirection: Ensure that stty is run directly in a terminal without piping input from another source.

- Check Context: Verify the context in which stty is being executed. If it’s a script, ensure the script is run in an interactive shell or that the stdin is properly connected to a terminal.

- Using 'script' command: The script command can be used to create a pseudo-terminal for commands that require one. For example:

script -c "your_command_with_stty" /dev/null

This starts a new shell and runs the command within it, simulating a terminal environment.

- Conditional Execution: Check if a terminal is attached before running stty:

if [ -t 0 ]; then
  stty -a
else
  echo "Not running in a terminal."
fi

The -t 0 option checks if file descriptor 0 (stdin) is associated with a terminal.

- Adjust SSH Configuration: For SSH issues, ensure that you're allocating a pseudo-terminal. You can force TTY allocation with the -t option:

ssh -t user@host "your_command_with_stty"

4. Specific Examples and Fixes:

- Jenkins or CI/CD: If this error occurs in a CI/CD pipeline like Jenkins, ensure that the build environment provides a terminal. Some CI systems may require explicit configuration to enable TTY allocation for build jobs.

- Automated Scripts: Review your automated scripts and make sure they aren't inadvertently redirecting stdin when calling stty.

By understanding the root cause—namely, that stty requires a valid terminal device to be associated with stdin—you can better diagnose and resolve this error in macOS iTerm or other terminal environments. Remember to check the context in which stty is being called and ensure it has a proper terminal attached.

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