What’s actually slowing this PC down?
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First work out whether the text is ordinary output, a message sent to your terminal, or characters being inserted into terminal input. These have different causes and fixes: on Linux, stty tostop can stop background jobs that write to the terminal, while message-permission controls such as mesg n address certain user-to-user terminal messages. Neither is a fix for characters being injected into a command line.
Identify what the terminal is doing
Watch what happens to the text before changing settings. Text that appears on screen may be output from a background job or a terminal-messaging utility. Text that appears inside the command line, where the shell may accept it as input, is a different and more serious symptom. A visual interruption alone does not establish that anyone has compromised the system.
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- Output: text appears in the terminal display, but is not entered as part of your command.
- A terminal message: another user or process sends a message through a facility such as
write. POSIX describeswriteas sending text to a recipient’s terminal. - Input insertion: characters appear in the terminal’s input stream or command line. On Linux, the
TIOCSTIioctl is one mechanism documented for inserting a byte into a terminal’s input queue.
If a command line is being altered, avoid pressing Enter until you have checked what is there. Do not treat a screen-only interruption as proof of input injection.
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Inspect terminal settings before changing them
In the affected terminal, run:
stty -a
This reports the terminal’s current settings. POSIX specifies that stty can report terminal characteristics and that -a displays all settings. It uses the terminal connected to standard input, rather than relying on an output-only path to address a terminal device. On shared systems, do not casually run commands intended to change another user’s terminal.
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Stop background jobs from writing to the terminal
If the interruption is output from a background job and you want the terminal to stop those writes, try this in the affected shell:
stty tostop
On Linux, this enables the terminal’s TOSTOP behavior: background jobs that try to write to the terminal are stopped. POSIX describes the behavior as sending SIGTTOU to the background process group. That can pause a job rather than quietly discard its output, so the job may need ordinary job-control handling. This setting does not block foreground program output, deny terminal messages, or prevent input insertion.
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To restore the usual behavior for background output in that terminal, run:
stty -tostop
These commands change terminal settings for the current terminal; they are not a system-wide security control. If you are unsure whether the output is from a background job, inspect the setting and observe which job is producing the text before leaving tostop enabled.
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Deny terminal messages when that is the source
POSIX allows a user to deny permission to receive a message sent with write. On many Unix-like systems, mesg n is the conventional command for denying such messages in the current session:
mesg n
Availability, exact behavior, and permission policy vary by system; check the local mesg manual page or ask the administrator on a shared machine. This affects message-receiving permission, not arbitrary output from programs running in the terminal and not characters inserted into shell input.
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Treat inserted command-line characters as a separate problem
If text is appearing as keystrokes or being added to shell input, neither stty tostop nor mesg n addresses the underlying mechanism. Check which processes are running under your account and which processes can access the terminal device, and involve the system administrator if you do not recognize the activity.
Linux documents TIOCSTI as a way to insert a byte into a terminal’s input queue. The Linux man-pages TIOCSTI page, version 6.19 dated February 8, 2026, notes that on Linux 6.2 and later this operation may require CAP_SYS_ADMIN when dev.tty.legacy_tiocsti is false. That is a Linux-specific, configurable rule; it is not a guarantee about every Linux configuration, Unix-like system, or terminal emulator.
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- If the input may execute an unsafe command, do not submit it. Move to a fresh terminal or trusted console if practical.
- On a shared system, ask the administrator to review processes and terminal access rather than changing device permissions yourself.
- If a compromise seems plausible, preserve relevant details and seek help before killing processes or making changes that could obscure what happened.
Choose the control that matches the symptom
| What you observe | Possible source | Relevant action | What it does not address |
|---|---|---|---|
| Output from a background job appears in the terminal | A background process writing to its controlling terminal | On Linux, use stty tostop; undo it with stty -tostop. |
Foreground output, terminal messages, or input insertion |
| A message arrives through a terminal messaging facility | A user or process using write or a similar facility |
Check local policy and syntax; mesg n is conventional on many Unix-like systems. |
General program output or input insertion |
| Unexpected characters enter a command line | Potential access to terminal input; Linux documents TIOCSTI as one possible mechanism |
Inspect processes and terminal access; consult an administrator and system security controls. | There is no general-purpose fix established here; output and message controls are not remedies |
Terminal behavior depends on the operating system, kernel configuration, utilities, and whether the session is shared. A process with a controlling terminal can refer to it through /dev/tty on systems that provide that device, but the identity of the process causing a particular interruption must be determined on that system.
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