如何在 bash 脚本中多次使用带有“-”（破折号）的标准输入？

Question

I'm writing a bash script whose input can be given through pipe, for this I use "-" but it can be used once. I tried to save it in a variable with input=$(cat -) but I am worried that if the input file is very large it may have problems. I would like to know the best way to use the input that "-" gives me.

The idea of the script is to do something like this:

#!/bin/bash
a=(cat - | grep -c "whatever")
b=(cat - | grep -c "whatever")
c=$(echo "($a+$b)" | bc)
echo "$c"

Answer 1

By default, cat reads its standard input. So "-" is not needed.

If you need to "grep" two regular expressions from the input, you can use grep without cat as it reads its standard input by default as well.

If the goal is to count the number of occurrences of two different patterns in the input, the equivalent of what you are trying to do could be something like:

#!/bin/bash
grep -c -E '(whatever1|whatever2)'

As pointed out in the comments, if the requirement is also to take in account multiple occurrences of the patterns in the lines and both patterns in the same lines, the "-o" option of grep can be used:

-o, --only-matching
Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line

With such an option, the output of grep is as many lines as there are matching patterns. So, the enhanced code counts those lines with wc -l and displays the result:

#!/bin/bash
grep -o -E '(whatever1|whatever2)' | wc -l