[SOLVED] Problem printing column on remote host via script using awk

keif · 10-25-2013, 08:34 AM

Hello everyone,

Ultimately what I'm trying to do is run ifconfig to print out the physical hardware, then grab the MAC address and put it into a file in /tmp (So later in the script I can stick the MAC into /etc/sysconfig/network-scripts/ifcfg-eth0, eth1 and eth2).

Code:

#!/bin/sh
ssh root@10.201.7.183 "\
ifconfig eth0 > /tmp/eth0file; \
ifconfig eth1 > /tmp/eth1file; \
ifconfig eth2 > /dev/null 2>&1 > /tmp/eth2file; \
awk '{print \$5}' /tmp/eth0file | head -1 > /tmp/eth0file; \
awk '{print \$5}' /tmp/eth1file | head -1 > /tmp/eth1file; \
awk '{print \$5}' /tmp/eth2file | head -1 > /tmp/eth2file"

In the above method, the first part works (Sending ifconfig to /tmp/ethfile. But when I try to separate the MAC from the rest of the ifconfig information I keep ending up with an empty file.

The funny thing is when I log on to the remote host and do this from the local command line it works fine. I think awk is causing the problem. I've had problems with using awk remotely this way before. But I could be wrong.

I wonder if anyone can recommend an alternative to awk in this case. Or perhaps my entire method for putting the MAC hardware into the eth files can be done better. Thanks!

druuna · 10-25-2013, 09:01 AM

You mention that above works locally, which I doubt very much.

This construct will not work, you'll end up with an empty file:

Code:

awk '{ print $5 }' /tmp/eth0file | head -1 > /tmp/eth0file

The reason for this is that bash will look at the whole command, sees that the output is redirected to a file called eth0file and creates this file (now empty). Then the awk part is executed, which uses the same file as input, but it is now empty.

As a rule: Never use the same file name as input and output.

A possible solution for your problem:

Code:

#/bin/bash
ssh root@10.201.7.183 "\
ifconfig eth0 | awk '/eth0/ { print \$5 }' > /tmp/eth0file ; \
ifconfig eth1 | awk '/eth1/ { print \$5 }' > /tmp/eth1file ; \
ifconfig eth2 | awk '/eth2/ { print \$5 }' > /tmp/eth2file"

keif · 10-25-2013, 10:00 AM

Quote:

Originally Posted by druuna

You mention that above works locally, which I doubt very much.

This construct will not work, you'll end up with an empty file:

Code:

awk '{ print $5 }' /tmp/eth0file | head -1 > /tmp/eth0file

The reason for this is that bash will look at the whole command, sees that the output is redirected to a file called eth0file and creates this file (now empty). Then the awk part is executed, which uses the same file as input, but it is now empty.

As a rule: Never use the same file name as input and output.

A possible solution for your problem:

Code:

#/bin/bash
ssh root@10.201.7.183 "\
ifconfig eth0 | awk '/eth0/ { print \$5 }' > /tmp/eth0file ; \
ifconfig eth1 | awk '/eth1/ { print \$5 }' > /tmp/eth1file ; \
ifconfig eth2 | awk '/eth2/ { print \$5 }' > /tmp/eth2file"

Thank you druuna. This was a much better solution and it works great.

Also, thanks for the new rule I'll add to my best practices list