How can I use parameters in a bash script as subject text to grep?

Question

I have a simple bash script, which receives a file with subject text, and processes line by line.
If the line begins with certain characters - then run block of compound commands. I am trying to use grep to test the lines for patterns (but would accept other suggestions).

file: matcher.sh

#!/usr/bin/env bash

for i in "$@"
do
    if grep "^M" $i   # I want grep to "assume" $i was a file
                      # and test if the pattern "^M" is present 

    then
        echo "This line started with an M: "$i 
        # command 1
        # command 2
        # etc
    fi
done

Subject_text.txt

D       bar
M       shell_test.sh
M       another_file

Then run script with

cat subject_text.txt | xargs --delimiter="\n" ./matcher.sh

How can I get grep to treat each iteration $i through the parameter list as if $i were a file?

Answer 1

You can read the file Subject_text.txt in a loop and feed matcher.sh with the name of the file to check:

while IFS= read -r _ file_name
do
   ./matcher.sh "$file_name"
done < "Subject_text.txt"

However, now that I see, you are using matcher.sh over every line. Note that calling a script with every single line as parameter is a bit overkilling.

What about looping over the file normally and performing the grep ?

#!/usr/bin/env bash

file=$1

while IFS= read -r line
do
    if grep "^M" <<< "$line"   # I want grep to "assume" $i was a file
                           # and test if the pattern "^M" is present 

    then
        echo "This line started with an M: $i"
        # command 1
        # command 2
        # etc
    fi
done < "$file"