3 ms·

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"

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? Thanks.

LukeShu 13y ago
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
sigil 13y ago
> 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 $line done PS. You should almost always use `find` instead of `ls` in shell scripts. Given a pattern, `ls` will exit non-zero if nothing matches it, and you should be treating non-zero exits like you would exceptions in other languages.
- joseph 13y ago
  One thing to be careful of when doing "while read..." is that a new shell is started on each iteration, so you cannot for example set a variable within the loop that you can use later in the script, as its value will be lost when the shell process exits.
  - sigil 13y ago
    > a new shell is started on each iteration This is not actually true. printf "\n\n\n" | while read i; do a="x$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
    - joseph 13y ago
      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.

LukeShu 13y ago

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

sigil 13y ago

> 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.

joseph 13y ago
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.
- sigil 13y ago
  > a new shell is started on each iteration This is not actually true. 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
  - joseph 13y ago
    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.