My understanding is that a pipe in shell like a | b redirects b's stdin to a's stdout. However, I tried ls | less < somefile, the behavior is very strange. less shows the output of ls concatenated by the content of somefile.
Could someone explain this phenomenon?
Your understanding is not quite correct. In a | b the stdout output of process a connected through a pipe to stdin of process b. The problem with your code is that with an additional redirection from somefile to process b you will use two different methods at the same time to connect to stdin of process b. Don't do that! The question is; what do you try to achieve (with such a construct) in the first place?
You don't say what shell are you using. From the behaviour you are describing it's likely zsh. If you have a look in its man page you would notice how redirections are handled.
Note that a pipe is an implicit redirection; thus
cat bar | sort <foois equivalent to
cat bar foo | sort(note the order of the inputs).
Otherwise, regular behaviour e.g., that one of bash is that each redirection replaces the previous redirection for that file descriptor.
zsh man page continues:
If the MULTIOS option is unset, each redirection replaces the previous redirection for that file descriptor. However, all files redirected to are actually opened, so
echo foo > bar > bazwhen MULTIOS is unset will truncate bar, and write `foo' into baz.
because you are taking input of less from somefile.
it works like this
1 -> execute ls , as you have used pipe , it will pipe stdout to stdin of next command : this is what you are telling
2 -> it will look second command.. it is less < somefile
as soon as it sees '<' redirection it will change stdin again to the file... Hence your previous change because of '|' is getting lost.
well, you can say it like this : less have taken input from file , it has nothing to do with ls's output.
