Loops: for

A for loop takes a list of words and runs a block once per word, with a variable holding the current one. The list can be typed by hand, produced by a wildcard, or generated by a command inside $( ). That is the whole feature, and it covers most of the repetition you will ever need.

~% for color in red green blue; do echo "Color: $color"; done
Color: red
Color: green
Color: blue
~% for i in $(seq 1 3); do touch "report-$i.txt"; done
~% ls
report-1.txt  report-2.txt  report-3.txt
~% for f in *.txt; do echo "$f has $(wc -l < "$f") lines"; done
report-1.txt has 0 lines
report-2.txt has 0 lines
report-3.txt has 0 lines

On one line the parts are separated by ;: for var in list; do commands; done. In a script you spread them over lines and the semicolons disappear. seq 1 3 prints 1 2 3 on separate lines; seq 0 5 20 counts in steps of five, and seq -w 1 10 pads with zeros so names sort properly: 01, 02, …

Looping over files

for f in *.txt is the idiomatic way to visit files. The shell expands the wildcard before the loop starts, so each file name is one word even when it contains spaces. That safety is lost the moment you write for f in $(ls): the output of ls is split on every space, so my notes.txt becomes two useless words. Use globs, not ls, and quote "$f" wherever you use it.

~% cd lab/l85/pics && ls
holiday 1.jpeg  holiday 2.jpeg  logo.jpeg
pics% for f in *.jpeg; do mv "$f" "${f%.jpeg}.jpg"; done
pics% ls
holiday 1.jpg  holiday 2.jpg  logo.jpg
pics% for f in *.png; do echo "found $f"; done
found *.png

${f%.jpeg} is the value of f with the shortest match of .jpeg removed from the end; ${f#prefix} does the same from the front, and ${f##*/} strips everything up to the last slash, like basename. The last command shows a classic surprise: when a pattern matches nothing, the shell leaves it as literal text, so the loop runs once with f equal to *.png. Guard against it with [ -e "$f" ] || continue as the first line of the body.

break and continue

continue skips to the next item; break leaves the loop altogether. Both are commands, so you combine them with tests: [ -d "$f" ] && continue skips directories, and [ "$n" -gt 100 ] && break stops after a limit. The loop as a whole has an exit status too, that of its last command, so you can follow done with && echo finished.