(gdb) thread apply all bt
Thread 12 (Thread 0x7f7fe2116700 (LWP 5466)):
#0  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86
#1  0x0000000000425358 in ?? ()
...
Thread 1 (Thread 0x7f7feabc27c0 (LWP 5465)):
#0  0x00007f7fe76c5203 in select () at ../sysdeps/unix/syscall-template.S:82

$ gdbbt() {
  tmp=$(tempfile)
  echo thread apply all bt >"$tmp"
  gdb -batch -nx -q -x "$tmp" -p "$1"
  rm -f "$tmp"
}
$ gdbbt $(pidof $SHELL)

pstack <pid>