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Is there any faster way to grep the pattern or to increase the speed of while loop?

this loop is taking time because lacks of entry in main_file. 

while read line;do  
         cat main_file | grep "$line";
done<file

cat file

pattern_1
pattern_2

cat main_file
  main  pattern_1
  main  pattern_2
  main pattern_2

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Answer

Your current approach is very inefficient – the whole loop could be done in a single grep, with the -f option.

grep -Fxf file main_file
  • -F treats lines in file as strings, not patterns
  • -x looks for exact matching line (if that is what you want)
  • -f file reads the lines from file and looks for them in main_file

The above approach will work well as long as the files are small. For larger files, use awk:

awk 'FNR==NR {hash[$1]; next} $2 in hash' file main_file

For details, look at this post – it had other solutions as well:

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