Why does the redirection symbol change the behavior of ls?

Question

So, I have always had doubts about how redirection works in the following situations:

1. I type "ls" and all the filenames are separated by white spaces:

   test$ touch a b c
   test$ ls
   a b c
   

2. I use a ">" to redirect STDOUT to a file:

   test$ ls > ls.txt
   test$ cat ls.txt 
   a
   b
   c
   ls.txt
   

It is interesting to see that the format changes, with the filenames separated by newline characters. It seems that the output is generated by ls -1.

Why is the output in the latter case different from that in the former case? Can ls actually see the ">" symbol so it changes its behavior?

Answer 1

ls tests its output stream to see whether it is a terminal, and it modifies its behavior depending on that.

This is documented; the man page for ls documents several things that depend on whether the output is a terminal:

• If the output is a terminal, -C (for multi-column output) is a default, otherwise -1 (one-column) is a default.
• If -l or -s is used and the output is a terminal, a sum for all file sizes or blocks, respectively, is printed on a line before the listing.
• If the output is a terminal, -q is a default. This prints non-graphic characters as “?”. Otherwise, -v and -w are defaults. I am a bit unclear on the difference between -v and -w. The documentation I have says -v forces “unedited printing of non-graphic characters” and -w forces “raw printing of non-printable characters.”

Answer 2

It cannot see the symbol (which is interpreted by the shell), but it can find out whether the output is going to a terminal.

In order to organize files nicely into the columns, ls needs to know the width of the terminal. When the output device is not a terminal, it just doesn't know how to format its output.

Another nice consequence of this behavior is that you can do things like ls | wc -l without worrying about multiple files on the same line. (You still have to worry about file names containing newlines, though.)