The following does not work on files with spaces according to the article:
for i in $(ls *.mp3); do
some command $i
done
So does that mean, that "for" will do something per word of the output of $, rather than per line of output of it?
What to do if I want to do something for every line? What for example if I really want the output of ls, find (or any other command you can put int he $()) and loop through that line per line, even if some output has spaces?
There are a couple of ways you can do it. Don't do this with `ls` though; there are better ways to get that info in scripts.
# Loops over lines, in a subshell
prints_lines | while read -r line; do
some_command "$line"
done
# Loops over lines, in the current shell
while read -r line; do
some_command "$line"
done < <(prints_lines)
# If for some reason you really want a for loop
oldIFS=$IFS
IFS=$'\n' lines=($(prints_lines))
IFS=$oldIFS
for line in "${lines[@]}"; do
some_command "$line"
done
> So does that mean, that "for" will do something per word of the output of $, rather than per line of output of it?
Correct. The argument to "for" is a list of words.
> What to do if I want to do something for every line?
Use a while loop.
find /some/dir/ -type f |
while read -r line; do
; # something with $line
done
PS. You should almost always use `find` instead of `ls` in shell scripts. Given a pattern, `ls` will exit non-zero if nothing matches it, and you should be treating non-zero exits like you would exceptions in other languages.
One thing to be careful of when doing "while read..." is that a new shell is started on each iteration, so you cannot for example set a variable within the loop that you can use later in the script, as its value will be lost when the shell process exits.
printf "\n\n\n" | while read i; do a="x$a"; echo "$a"; done
x
xx
xxx
The accumulator value even carries over after the while loop:
printf "\n\n\n" | ( while read i; do a="x$a"; echo "$a"; done ; echo "$a" )
x
xx
xxx
xxx
(Technically, whether or not the loop body is executed in a subshell may be implementation dependent. Haven't looked at the POSIX shell spec in a while, but I seem to remember an old ksh that actually used subshells. At any rate, none of the modern sh's and bash force a subshell.)
What is true, however, is that a pipeline will execute in a subshell. Maybe that's what you're getting at here, and it is an important caveat.
a=y; printf "\n\n\n" | while read i; do a="x$a"; echo "$a"; done; echo "$a"
xy
xxy
xxxy
y
Ah, ok. True, when I have had this issue it was after doing something like 'grep "pattern" file | while read ...'. I did not realize it was the pipe that caused this.
What to do if I want to do something for every line? What for example if I really want the output of ls, find (or any other command you can put int he $()) and loop through that line per line, even if some output has spaces?
Thanks.