# We have already seen simple ‹for› loops, even though we didn't
# quite understand variables. For loops are quite simple:

for var in x y z; do
    echo $var
done

# By now, you should understand what ‹echo $var› means, and the
# ‹for› loop simply means this: set ‹var› to ‹x›, run the body of
# the loop, then set it to ‹y›, run the loop again and then set it
# to ‹z› and repeat once more. The basic structure is like with
# ‹if›: you can use semicolons or newlines, as you prefer.

# Of course, the above form of a loop can be useful, but the things
# to iterate will often be files. You may remember that we can use
# «globs» (patterns) to match multiple files. This works
# universally, not just in a for statement:

echo *.mk
echo

for file in *.txt; do
    if test $file -nt make.txt; then
        echo "$file is newer than make.txt"
    else
        echo "$file is older than make.txt"
    fi
done

# The last form of a loop that I want to show you is a ‹while› loop.
# It is written like this:

echo
touch scratch.txt
while test $(wc -l < scratch.txt) -lt 5; do
    echo . >> scratch.txt
done
wc -l scratch.txt

# We can also pipe things to and from loops:

for i in 1 2 3 4 5; do
    echo .
done | wc -l

echo

# Especially useful (when piping things into loops) is the ‹while
# read› construct. The ‹read› command is provided by the shell to
# read inputs and put them into variables. ‹command | read var› is
# similar to ‹var=$(command)›. You can try running:
#
#     $ echo bla | read foo
#     $ echo $foo
#
# The loop version looks like this:

cat scratch.txt | while read line; do
    echo the line is this: $line
done

# That is all about loops. You can now go on to ‹2.txt›:
#
#     $ cat 2.txt