A script causes a parent script to stop

I have a third-party script that runs some complicated stuff. I run everything from an ssh session (the machine is a headless VM running Ubuntu 20.04). When I run it interactively, it runs normally. $ ./third-party-script.sh one $ ./third-party-script.sh two When I run it from another script, the parent script stops when the third-party script executes for the *second* time. $ cat my-script.sh #!/bin/sh ./third-party-script.sh one ./third-party-script.sh two $ $ ./my-script.sh + Stopped ./my-script.sh $ I can send my script to foreground and it continues normally $ fg $ What could be the reason of this behaviour and how can it be avoided? I cannot fix the third-party script or anything that it runs, I can only fix my script. (The original "my-script" is written in Python and uses os.system, I simplified it to a bare-bones shell script for this question). I tried running the script with nohup but it didn't help. Thanks to @ilkkachu, I have narrowed it down to the third-party script executing bash -ic some_command. So the minimal script that reproduces this in *my* environment is: #!/bin/sh bash -ic ls bash -ic ls sh -ic and bash -c both fix the problem. Unfortunately I cannot change the command. I need to *wrap* the third-party script somehow so that it stops doing this. I tried bash -icx instead, but this doesn't narrow down the problem any further. The second bash execution stops before printing any command. I tried to strace -f the whole thing and this is what comes in the end: [pid 3805453] ioctl(255, TIOCGPGRP, ) = 0 [pid 3805453] rt_sigaction(SIGTTIN, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f131f36a090}, {sa_handler=SIG_IGN, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f131f36a090}, 8) = 0 [pid 3805453] kill(0, SIGTTIN) = 0 [pid 3805321] 0x7ffcc2d45bfc, 0, NULL) = ? ERESTARTSYS (To be restarted if SA_RESTART is set) [pid 3805453] --- SIGTTIN {si_signo=SIGTTIN, si_code=SI_USER, si_pid=3805453, si_uid=4437} --- [pid 3805321] --- SIGTTIN {si_signo=SIGTTIN, si_code=SI_USER, si_pid=3805453, si_uid=4437} --- [pid 3805453] --- stopped by SIGTTIN --- [pid 3805321] --- stopped by SIGTTIN --- 3805453 is the *second* child bash running -ic ls and 3805321 is the top-level bash running my-script.sh. It is the only line with kill(0, SIGTTIN) in the strace log. The first child bash doesn't do anything like that, only the second one does.