The shell has loops and tests, and they read differently from a programming language because they operate on words and exit statuses. Once that is clear, a shell loop is a short way to do the same thing across a list of files.
About 1 min of reading
Loop over files and count lines.
Set up
Before you run it, say out loud what it will print. Then open the answer.
Loop
Before you run it, say out loud what it will print. Then open the answer.
A loop that runs one command per file is often one command with a list. grep over a glob or xargs does the work in one process, and the loop version is slower and harder to read.
A shell loop leaves its variable set after it finishes, which bites when the same name is reused later. Use a name that says what it holds, and the leftover value stops being a mystery.
for f in *.txt; do wc -l "$f"; done. The glob expands once, then the body runs per name. Quoting inside the loop is what keeps names with spaces working.
if command; then ... runs the command and branches on whether it succeeded. Test brackets are a program that returns a status. Both are the same mechanism: success is zero, failure is not.
while read -r line; do ... done < file processes a file line by line. The -r flag stops backslashes being eaten, which is a default worth setting without thinking.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.