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is there a way to do this?

cat somefile.txt | grep this_pattern OR grep that_pattern

I want display only certain lines from a large file however not just by one pattern.

For example

cat output.txt | grep "username" OR grep "connection-time" OR grep "disconnect-time"

This would conveniently give me something like this which is all I want if I could use multiple grep's based on a logical OR

username = bob
connection-time : <date & time>
disconnect-time : <date & time>
username = ron
connection-time : <date & time>
disconnect-time : <date & time>
username = jack
connection-time : <date & time>
disconnect-time : <date & time>
... and so on.
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  • 1
    Perhaps this will help grep with logic operators Commented Oct 29, 2024 at 17:42
  • yeah, a simple grep -e this -e that is all I needed, thx Commented Oct 29, 2024 at 17:48
  • What you asked for could be written grep -E 'username|(connection|disconnect)-time' but in general you might find it simpler to use awk than grep when you have multiple patterns to match since it uses compound conditions, not just individual regexps, e.g. awk '/this/ && /that/', awk '/this/ || /that/', awk '/this/ && (/foo/ || /bar/)', etc. Commented Nov 7, 2024 at 12:09

1 Answer 1

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2

This isn't so much about grep but about basic regular expression usage. Unless explicitly told not to do so by using -F, grep will interpret the patterns it is given as regular expressions. OR is a common feature of all regex flavors I am familiar with, you just do pattern1|pattern2. With BRE, the default flavor for grep, you need \|, but if you use -E or -P for extended or PCRE respectively, you can just use |. You also don't need cat:

grep 'username\|connection-time\|disconnect-time' somefile.txt

or

grep -E 'username|connection-time|disconnect-time' somefile.txt

or

grep -P 'username|connection-time|disconnect-time' somefile.txt
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