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I'm relatively new to Bash scripting but I'm very familiar with Python and many Bash commands. I was wondering if it's possible to use a command such as ifconfig eno1 and use the output as a condition to be used in an if statement. Basically, if you issue:

 ifconfig eno1

and you get an active IP, is there any way to save the output of that to a variable as a 1/0 or true/false, to be used in the following if statement example (assuming ifoutput is the variable name): 

 if [ "$ifoutput" == "$True"] then echo connection is active! fi

I thought of possibly calling a Python script that does essentially the same thing, however I was thinking I would run into the same issue; being no way to get an individual return value from the Python script to be used in the if statement. Please advise and please go easy on me -- as I said I am relatively new to Bash scripting! I already have methods of doing this strictly using Python, however I am trying to learn more about Bash scripting. Thanks in advance!

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  • you could write it to file using > filename.txt or pipe it to a different command, | nextCommand. Commented Aug 6, 2018 at 15:22

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ifconfig will return a numeric 8-bit value when it completes (like any *nix process that exits cleanly).

Typically zero (0) is used to indicate success, and another value is used to indicate failure. This return code is available as $?.

ifconfig will also output to stdout, for example:

eth0 Link encap:Ethernet HWaddr 02:42:ac:14:00:02 inet addr:172.20.0.2 Bcast:172.20.255.255 Mask:255.255.0.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:20430143 errors:0 dropped:0 overruns:0 frame:0 TX packets:20966475 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:46532669457 (46.5 GB) TX bytes:44114090048 (44.1 GB)

If you are trying to determine if ifconfig eth0 was successful (i.e: permissions depending, the interface exists), then you could do one of the following:

ifconfig eth0 if [ $? -eq 0 ]; then echo yay fi 
if ifconfig eth0; then echo yay fi 
ifconfig eth0 && echo yay

Hiding ifconfig's output can be achieved by appending a > /dev/null:

if ifconfig eth0 > /dev/null; then echo yay fi

You asked about printing the return code, and this is a gotcha for beginners...

You can simply do this:

ifconfig eth0 echo $?

But after calling on echo, the return code ($?) will be echo's return code, not ifconfig's... To deal with that, just assign it to a temporary variable:

ifconfig eth0 RET=$? echo ${RET} if [ ${RET} -eq 0 ]; then echo yay fi

Note: [ is a built-in command that can evaluate expressions, the square braces ([ / ]) are not actually part of the if syntax:

$ help [ [: [ arg... ] Evaluate conditional expression. This is a synonym for the "test" builtin, but the last argument must be a literal `]', to match the opening `['.

If you are actually trying to get the IP address of an interface, then you might want to use a pipeline, like one of the following (there are many ways to achieve the same result):

ifconfig eth0 | grep -Eom1 'inet addr:([0-9]{1,3}\.){3}[0-9]{1,3}' | cut -d: -f2 ifconfig eth0 | sed -re '/inet addr:/!d;s/^[^:]+://;s/ .*$//'
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  • Thank you! Initially I had thought of using the exit code, but since I am still fairly new , I didn't realize the exit codes would be similar to what i'm used to working with in Python! Thanks for the additional information as well! I feel confident I can carry out my project now! is there any way to show the exit code upon execution? Commented Aug 6, 2018 at 15:38
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    No problem - and of course! echo $?... see my edit for explanation of a gotcha though. Commented Aug 6, 2018 at 15:40
  • tldp.org/guides.html -> Advanced Bash-Scripting Guide - has a lot of information regarding Bash and scripting. Commented Aug 6, 2018 at 18:49

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