How to place all text from a file into the search pattern of grep?

Question

I am using grep like this:

grep '^[abcd]*$' file.txt

I have a file, list.txt , containing a list like this:

potato
orange
carrot

How can I send all of the lines from list.txt to inside the search pattern of grep , the equivalent of typing this:

grep '^[potatoorangecarrot]*$' file.txt

All of the lines in list.txt are put together and sent to grep as one long search.

How can I place all of the text from a file into the search pattern of grep?

Answer 1

Like this:

string="^[$(tr -d '\n' < list.txt)]*$"
echo $string

Then

grep "$string" file.txt

The $() bit says " take the output of running the command within its parentheses ". The tr command says to delete ( -d ) all linefeeds from file list.txt .