Ping another computer if ping to 1st computer fails

[Linux_Newbie_2007]

Well, i'm new to Linux and I'm trying to do the following.

I have 2 computers and i can ping them both but just as an experiment I want to disconnect Computer A (which always gets pinged first). This will fail as it is disconnected and then I want it to go and ping Computer B. Eventually I want it to actually connect to the computer that is online.

Eg...something like (and yes I know the script is wrong I'm still learning so hopefully you can help)

#start of programme
ping A

if A not responding
then ping B
if B not responding
echo Communications down!
end if
end if

#end of programme

Also is it possible for me to do something like if ping A is successful then connect to A? and the same for B....

Thanks in advance.

[apoorv_khurasia]

Ping replies are very verbose and we can capture them and run grep on them.

so something on the line of...

ping some_machine | grep "network host unreachable"

[Linux_Newbie_2007]

I found that using grep caused my shell to display nothing. it didnt even show that it was attempting to ping the pc.....would you mind telling me what grep exactly does?

[apoorv_khurasia]

Grep is a regular expression matching utility. You can read the man page for more details. In this case it will display only lines that contain "network host unreachable".

[anomie]

#start of programme
ping A

if A not responding
then ping B
if B not responding
echo Communications down!
end if
end if

#end of programme

This seems strange to me. Who cares if you can ping to box A or box B? It sounds like what you presumably care about is whether you can connect to a certain (tcp?) port on those boxes. Unless I've read your post wrong, that is the true test.

Also is it possible for me to do something like if ping A is successful then connect to A? and the same for B....

Meaning what? Connect how? What client/daemon? Maybe you should explain in clear detail what you're trying to do.

[anomie]

I'd envision the program flow to look something like this:

1. Can I connect to service port on box A? If so, fire up client and connect, and we're done. If not, continue...
2. Can I connect to service port on box B? If so, fire up client and connect, and we're done. If not, continue...
3. Fail noisily.

[bigtomrodney]

I found that using grep caused my shell to display nothing. it didnt even show that it was attempting to ping the pc.....would you mind telling me what grep exactly does?

Make sure you specify a count for ping or it will run indefinitely thus preventing grep from working.

 ping -c 1 host

[secondmouse]

#start of programme
ping A

if A not responding
then ping B
if B not responding
echo Communications down!
end if
end if

#end of programme

Also is it possible for me to do something like if ping A is successful then connect to A? and the same for B....

Thanks in advance.

Try this:
test -z "`ping -c 1 -w 1 IPA | grep 100%`" || test -z "`ping -c 1 -w 1 IPB | grep 100%`" || echo "down"

More complicated version:

#!/bin/bash
IPA=....
IPB=....
if [ -n "`ping -c 1 -w 1 $IPA | grep 100%`" ]; then
    #A down, try B
    if [ -n "`ping -c 1 -w 1 $IPB | grep 100%`" ]; then
        echo "connection down!"
    else
        echo "connecting to B"   
        #to do: connecting to B
    fi
else
        echo "connecting to A"   
        #to do: connecting to A
fi

[NeoIce]

instead of using grep, you could also use return codes, since ping returns 0 if it gets 100% response, 1 if it drops some (or all) and 2 of it breaks alltogether.

$? is the shell variable representing the return code

so things would look sort of like:

ping -c 3 hostA
if $? == 0
      connect hostA
else
      ping -c 3 hostB
      if $? == 0
           connect hostB
      else
           echo "OMG YOU BROKE IT!"

sorry about the pseudo-code, but the important part is $?