Pfthb

I have a bunch of strings that I have to fetch the 'port_num' from -

"76 : client=new; tags=circ, LINK; port_num=switch01; far_port=Gi1/0"

The word might be in a different place in the string and it might be a different length, but it always says 'port_num=' before it and ';' after it...

I only want this bit- 'switch01'

Currently I use-

| grep -Eo 'port_num=.+' | cut -d"=" -f2 | cut -d";" -f1'

But there has got to be a better way

You can try grep -oP '(?<=port_num=).+(?=;)', if you run this:

echo "76 : client=new; tags=circ, LINK; port_num=switch01; far_port=Gi1/0" 
| grep -oP '(?<=port_num=).+(?=;)'

result will be:

switch01

Updated answer: grep -oP '(?<=port_num=)[^;]+(?=;)'

This is what I would use:

... | grep -E 'port_num=.+' | sed 's/^.*port_num=([^;]*).*$/1/'

This works with or without the -o on grep, and the availability of -P will depend on the version of grep you have. (e.g., my grep does not have it). I'm not saying the other answers that rely on -P aren't any good -- they look fine to me. But grep -P will be less portable.

IMHO, piping grep with sed allows each utility to do what it specializes in -- grep is for selecting lines, sed is for modifying lines.

This can be done in a simple sed command:

s="76 : client=new; tags=circ, LINK; port_num=switch01; far_port=Gi1/0"
sed 's/.*port_num=([^;]*);.*/1/' <<< "$s"

switch01

... | grep -Po 'port_num.+(?=;)'

This uses grep's Perl Compatible Regular Expression (PCRE) syntax. The (?=;) is a look-ahead assertion which looks for a match with ";" but doesn't include it in the matched output.

This produces:
port_num=switch01

As @Vladimir Kovpak noted, if you want to exclude the "port_num=" string from this output, add a look-behind assertion:

... | grep -Po '(?<=port_num).+(?=;)'