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Using mythshutdown to block shutdown
Mythshutdown can be used to block an automatic shutdown. Since 0.21 it is possible to call mythshutdown --lock from each script so that shutdown will remain blocked until the last script has terminated and called mythshutdown --unlock.
# Unlock if the script is terminated for any reason
trap "/usr/local/bin/mythshutdown --unlock" 0 SIGHUP SIGINT SIGQUIT SIGABRT SIGKILL SIGALRM SIGSEGV SIGTERM
# Lock shutdown
# Do script stuff
See, also, Mythwelcome.