Aqui está a maneira como eu "aguardo a queda da rede e faço algo":
#!/bin/bash
# Monitor for the net going down, Do The Next Thing (from a config file)
# to bring the net up. Implicit assumpition that The Next Thing fixes it
# may be a problem.
# Walt Sullivan
# Note: places where you may need to adjust things to match
# your environment/taste are marked "#Adjust"
# determine my name
me=$0
me=${me##*/}
# my variables
debug=0 # set via --debug
verbose=0 # set via --verbose
original="" # set the <config.file>
original_update=0 # time <config.file> was last modified
config="/var/tmp/${me}.$$.config" # my writable copy of config
result="" # temporary use
# $ dpkg -S $(type -p nm-online)
# network-manager: /usr/bin/nm-online
# $ dpkg -S $(type -p ip)
# iproute2: /sbin/ip
# man ip-link;man ip-monitor;man ip-address;man 7 regex
# -h or --help or something's wrong in here
help () {
echo "${me} [-h|--help] [-v|--verbose] <config.file> " >&2
echo "" >&2
echo "Monitor the wireless network, and when it goes down, Do The" >&2
echo "Next Thing (as specified by the <config.file>), to bring" >&2
echo "the wireless net up." >&2
echo "" >&2
echo "The <config.file> contains #comments, blank lines, AND" >&2
echo "single line commands, of your choice, to correct the" >&2
echo "wireless network down condition. The first command in the" >&2
echo "<config.file> will be executed the first time the net goes" >&2
echo "down (or if the net is down when ${me} begins), the second" >&2
echo "command will be executed the next time the net goes down," >&2
echo "and so forth, wrapping around at the end. The number of" >&2
echo "single line commands in the <config.file> is unlimited." >&2
exit 2
}
function flip () {
# Return the first non-blank, non #comment line,
# and move that line (and all preceeding blank and #comment
# lines) to the end of (our copy of) the config file.
#
# ed pattern includes "/", but not "~" or "."
#Adjust the ed pattern in both places and in countconfiglines
ed --quiet "$config" <<EndOfEd
/^[A-Za-z0-9\/]/
1,.t$
1,/^[A-Za-z0-9\/]/d
wq
EndOfEd
}
function countconfiglines () {
# return number of config file lines
#Adjust must match the ed pattern in flip()
configfile="$1"
egrep --count '^[[:alnum:]/]' "$configfile"
}
function up-to-date () {
# updates configuration file if necessary
new_update="$(/usr/bin/stat --format='%Y' $original )"
if [[ "$new_update" -ne "$original_update" ]] ; then
if [[ $(countconfiglines "$original") -eq 0 ]] ; then
echo "Invalid configuration in $original" >&2
exit 4
else
/bin/cp --force "$original" "$config"
original_update="$new_update"
fi
fi
}
function netstate () {
# Return network state as "UP" or "DOWN"
#Adjust how you decide net is UP/DOWN
ip link show | egrep -q 'UP,LOWER_UP.* state UP'
if [[ $? -eq 0 ]] ; then
echo "UP"
else
echo "DOWN"
fi
}
# Execution begins here
# parse the args with getopt, adapted from
# /usr/share/doc/util-linux/examples/getopt-parse.bash
TEMP='getopt -o dhv --long debug,help,verbose \
-n "${me}" -- "$@"'
if [[ $? != 0 ]] ; then echo "${me} --help for help." >&2 ; exit 1 ; fi
# Note the quotes around '$TEMP': they are essential!
eval set -- "$TEMP"
while true ; do
case "$1" in
-d|--debug) debug=1; shift;;
-h|--help) help; shift;;
-v|--verbose) verbose=1; shift;;
--) shift; break;;
*) echo "Internal error! ${me} --help for help";exit 1;;
esac
done
# Did we get the <config.file>?
original="$1"
shift
[[ -z "$original" ]] && \
(echo "Missing config file ${me} -h for help" >&2 ; exit 1)
# If there are more parameters, confusion exists in the mind of the caller
[[ "$#" -ne 0 ]] && help
[[ -r "$original" ]] || (echo "${me}:Cannot read $original" >&2
exit 2)
if [[ $(countconfiglines "$original" ) -eq 0 ]] ; then
echo "${me}:Invalid configuration in $original" >&2
exit 4
else
# watch for changes, record "%Y time of last modification,
# seconds since Epoch"
original_update="$(/usr/bin/stat --format='%Y' $original )"
# make a writeable copy for our use, and clean it up at the end
# unless $debug
[[ $debug -ne 0 ]] || trap "/bin/rm -f $config" EXIT
/bin/cp --force "$original" "$config"
fi
# if the net is down, Do The Next Thing right away
#Adjust: how you decide the net is up or down?
[[ "$(netstate)" = "DOWN" ]] && \
( result="$(flip)";
[[ $verbose ]] && echo "$(date):$result" >&2; \
eval "$result" )
# Wait for the net to go down, then Do The Next Thing
#Adjust find a better way to watch for net down
ip monitor address | \
egrep --line-buffered \
'^Deleted [[:digit:]]+: [[:alnum:]]+[[:space:]]+inet[[:space:]].* scope global ' | \
while read line ; do
up-to-date
result="$(flip)"
[[ $verbose -eq 1 ]] && echo "$(date):$result" >&2
# Here is where "The Next Thing" is Done
eval "$result"
done
# we never exit the while loop, until the world ends.
exit 0