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bash

Some notes on bash.

Ignore Host Keys

Sometimes it can be useful to ignore host key verification when using ssh. For example, you may be connecting to VMs that generate new keys on startup, or occassionally re-use the same IP addresses (when connecting via IP rather than hostname). The following alias can be used as a quick workaround:

alias ssh-ignore='ssh -o "UserKnownHostsFile=/dev/null" -o "StrictHostKeyChecking=no"'
$ ssh-ignore user@192.168.99.101

Wake on LAN

Why bother getting up to power on the file server?

wake.sh 
mac=$1
mac=$(echo $mac |sed 's/[: -]//g')
broadcast=255.255.255.255
port=9

# FF FF FF FF FF FF
# MAC * 16
packet=$(
    printf "F%.0s" {1..12}
    printf "$mac%.0s" {1..16}
)

# hex encode
packet=$(echo $packet |sed -e 's/../\\x&/g')

# broadcast
echo -e $packet |nc -w1 -u -b $broadcast $port

echo "12:34:56:78:9A:BC" > ~/fileserver.mac
./wake.sh $(cat fileserver.mac)

Time Zone Conversion

While there are several very useful online tools to convert time stamps, I wanted something that doesn't require an Internet connection and helps quickly convert between some time zones I find myself working with regularly. It's not particularly elegant, but gets the job done.

Sample output:

$ ./tz.sh 13:37 CET
Sat Feb 20 13:37 CET
====================
Sat Feb 20 23:37 AEDT
Sat Feb 20 18:07 IST
Sat Feb 20 12:37 UTC
Sat Feb 20 07:37 EST
Sat Feb 20 06:37 CST
Sat Feb 20 04:37 PST
tz.sh 
#!/bin/bash

AEST="Australia/Sydney"
IST="Asia/Kolkata"
CET="Europe/Stockholm"
UTC="UTC"
EST="America/New_York"
CST="America/Chicago"
PST="America/Los_Angeles"

TZONES="${AEST} ${IST} ${CET} ${UTC} ${EST} ${CST} ${PST}"
FORMAT="+%a %b %d %H:%M %Z"

if [ "$1" ] ; then
    TIME=$1
else
    TIME=$(date "${FORMAT}")
fi

case ${2^^} in
    AEST)
        TIME_ZONE=$AEST ;;
    IST)
        TIME_ZONE=$IST ;;
    CET)
        TIME_ZONE=$CET ;;
    EST)
        TIME_ZONE=$EST ;;
    CST)
        TIME_ZONE=$CST ;;
    PST)
        TIME_ZONE=$PST ;;
    *)
        TIME_ZONE=$UTC ;;
esac

print_time()
{
    TIME_IN=$1
    TZ_OUT=$2
    TZ=${TZ_OUT} date "${FORMAT}" --date="${TIME_IN}"
}

TIME_IN=$(TZ=${TIME_ZONE} date "${FORMAT}" --date="${TIME}")
echo $TIME_IN
printf "=%.0s" {1..20}
echo

for zone in $TZONES ; do
    if [[ $zone == $TIME_ZONE ]] ; then
        :
    else
        print_time "${TIME_IN}" $zone
    fi
done