The file descriptor 255 is commonly seen in bash and other shells, and it usually refers to a backup of the original file descriptor for the terminal session.
Why Does FD 255 Exist?
- When you start a shell session, stdin (0), stdout (1), and stderr (2) are opened and connected to the terminal (
/dev/pts/24in your case). - Some shells (like Bash and Zsh) reserve FD 255 as a backup of the original stdin, stdout, and stderr so they can restore them if needed.
Practical Use of FD 255
1️⃣ Redirecting FD 255 for Debugging
Since FD 255 usually points to your terminal, you can use it for debugging even if stdout and stderr are redirected:
echo "This is normal output"
echo "This goes to the terminal" >&255Even if you do:
echo "Hidden output" > /tmp/output.logYou can still print to the terminal using:
echo "But this still prints!" >&2552️⃣ Using FD 255 in Subshells
When you run a script or command in a subshell, FD 255 allows the shell to restore the original output streams. Example:
bash -c 'echo "Hello" >&255'This prints “Hello” to the terminal because FD 255 is still linked to /dev/pts/24.
3️⃣ Closing FD 255
You can check if FD 255 is open:
ls -l /proc/$$/fd/255You can also close it manually:
exec 255>&-After this, trying to use >&255 will result in an error.
Conclusion
- FD 255 is used by Bash and some other shells to keep a reference to the original terminal session.
- It is useful for debugging, preserving terminal output in scripts, and handling redirections safely.
- You can close it manually, but the shell may recreate it when a new session starts.
Would you like to experiment with it using a script? 🚀