Loops: while and until

for needs its list up front. while does not: it runs a condition command, and as long as that command succeeds, it runs the body again. until is the mirror image, looping as long as the condition fails. Both end with done, and the condition is any command, usually [ ].

~% cat lab/l86/countdown.sh
#!/bin/sh
n=5
while [ "$n" -gt 0 ]; do
    echo "$n"
    n=$(( n - 1 ))
done
echo "Liftoff"
~% lab/l86/countdown.sh
5
4
3
2
1
Liftoff
~% n=0; until [ -e /tmp/ready ]; do n=$(( n + 1 )); [ "$n" -ge 3 ] && touch /tmp/ready; done; echo "waited $n turns"
waited 3 turns

Every while needs something in the body that eventually makes the condition false, here n=$(( n - 1 )). Forget it and the loop never ends; press Ctrl+C to kill it. Infinite loops on purpose are written while true; do …; done or while :; do …; done with a break inside, for example to retry a command until it works, with a sleep 1 between attempts so you do not burn the CPU.

Reading a file line by line

The most useful while in shell programming is while read -r line; do …; done < file. read takes one line from its input, stores it in line, and succeeds; at end of file it fails and the loop stops. The -r keeps backslashes literal. The redirection goes after done, so the whole loop reads from the file. Give read several names and the line is split on spaces: read -r name age puts the first word in name and the rest in age; set IFS=: in front to split on colons instead, handy for /etc/passwd.

~% cat lab/l86/numbers.txt
10
20
30
~% total=0
~% while read -r n; do total=$(( total + n )); done < lab/l86/numbers.txt
~% echo "total=$total"
total=60
~% total=0
~% cat lab/l86/numbers.txt | while read -r n; do total=$(( total + n )); done
~% echo "total=$total"
total=0

One more subtlety: read fails on the last line if the file does not end with a newline, so that line is silently skipped. Files you create with printf '…\n' or vi are fine; for files of unknown origin write while read -r l || [ -n "$l" ]; do to process the leftover.