Question

How to Prevent a User from Using CTRL+C or CTRL+Z to Stop a Script?

Posted March 6, 2021 353 views
Linux BasicsUbuntuLinux Commands

In some cases when writing a script you want to prevent the user from exiting the script with CTRL+C or CTRL+Z.

Here is how you could do that!

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1 answer

Hello,

Let’s take the following script as an example:

#!/bin/bash

function greeting(){
    echo "Enter your name"
    read name
    echo "Hi there ${name}"
}

# Call the function
greeting

With the above, we are expecting the user to enter their name and then greet them. However, the user could just press CTRL+C or CTRL+Z and exit the script.

To prevent that, we could use the trap command and trap specific term signals. For example, CTRL+Z sends the SIGTSTP signal, and CTRL+C sends SIGINT.

So if we wanted to prevent the user from exiting the script with CTRL+Z or CTRL+C, we could use the trap command and trap those signals:

#!/bin/bash

function greeting(){
    # Start the trap
    trap ' ' SIGTSTP SIGINT SIGTERM
    echo "Enter your name"
    read name
    echo "Hi there ${name}"
    # End the trap
    trap - SIGTSTP SIGINT SIGTERM
}

# Call the function
greeting

In case that you want to get a list of all term signals, you could use the following command:

trap -l

Output:

 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL       5) SIGTRAP
 6) SIGABRT      7) SIGBUS       8) SIGFPE       9) SIGKILL     10) SIGUSR1
11) SIGSEGV     12) SIGUSR2     13) SIGPIPE     14) SIGALRM     15) SIGTERM
16) SIGSTKFLT   17) SIGCHLD     18) SIGCONT     19) SIGSTOP     20) SIGTSTP
21) SIGTTIN     22) SIGTTOU     23) SIGURG      24) SIGXCPU     25) SIGXFSZ
26) SIGVTALRM   27) SIGPROF     28) SIGWINCH    29) SIGIO       30) SIGPWR
31) SIGSYS      34) SIGRTMIN    35) SIGRTMIN+1  36) SIGRTMIN+2  37) SIGRTMIN+3
38) SIGRTMIN+4  39) SIGRTMIN+5  40) SIGRTMIN+6  41) SIGRTMIN+7  42) SIGRTMIN+8
43) SIGRTMIN+9  44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9  56) SIGRTMAX-8  57) SIGRTMAX-7
58) SIGRTMAX-6  59) SIGRTMAX-5  60) SIGRTMAX-4  61) SIGRTMAX-3  62) SIGRTMAX-2
63) SIGRTMAX-1  64) SIGRTMAX

I personally prefer to use the numeric representation as follows:

...
    trap ' ' 2 15 20
    echo "Enter your name"
    read name
    echo "Hi there ${name}"
    # End the trap
    trap - 2 15 20
...

I hope that this helps!
Regards,
Bobby

  • Perfect, @bobbyiliev

    Another way to prevent the user from quitting the script using CTRL+C or CTRL+Z is stty

    To prevent this anyone can add:

    • stty intr ""
    • stty quit ""
    • stty susp undef

    Hope this helps!
    Regards,
    Alex