Pipe printf to ls in Bash?

Question

So I'm learning about pipes in bash and I found this pithy description:

A Unix pipe connects the STDOUT (standard output) file descriptor of the first process to the STDIN (standard input) of the second. What happens then is that when the first process writes to its STDOUT, that output can be immediately read (from STDIN) by the second process.

Source

Given this understanding, let's connect the STDOUT of printf to the STDIN of ls. For simplicity, print the parent directory (printf ..).

~/Desktop/pipes$ mkdir sub
~/Desktop/pipes$ ls
sub
~/Desktop/pipes$ cd sub
(no files)
~/Desktop/pipes/sub$ printf .. | ls
(no files)
~/Desktop/pipes/sub$

I want to be doing: ls .. but it seems that all I'm getting is ls. Why is this so? How can I ls the parent directory using pipes? Am I misunderstanding pipes?

`ls` doesn't read from standard input. So, `ls ..` is the way to get what you want here. `man ls` is your friend. — codeforester, Feb 15 '17 at 06:25
So is this simply a property of `ls`? Is my understanding of pipes otherwise correct? Is `ls ..` not taking the `..` argument from STDIN? — Kenny Worden, Feb 15 '17 at 06:27
Yes, that's a property of `ls`, it doesn't read arguments from stdin (and therefore a pipe won't have any effect). — Sir Athos, Feb 15 '17 at 08:26

score 3 · Accepted Answer · answered Feb 15 '17 at 09:12

Many programs don't read from stdin, not just ls. It is also possible that a program might not write to stdout either.

Here is a little experiment that might clarify things. Carry out these steps:

cat > file1
This is file1
^D

The ^D is you pressing <CTRL>+D, which is the default end-of-file. So, first we are calling the cat program and redirecting its stdout to file1. If you don't supply an input filename then it reads from stdin, so we type "This is file1".

Now do similar:

cat > file2
This is file2
^D

Now if you:

cat < file1

You get:

This is file1

what if you:

cat file1 | cat file2

or:

cat file2 < file1

Why? Because if you supply an input filename then the cat program does not read stdin, just like ls.

Now, how about:

cat - file1 < file2

By convention the - means a standard stream, stdin when reading or stdout when writing.

score 2 · Answer 2 · answered Feb 15 '17 at 06:30

2

The problem is ls does not read from stdin as you intended it to do. You need to use a tool that reads from the stdin like xargs and feed the read input to ls

printf "someSampleFolderOrFile" | xargs ls

xargs man page,

xargs - build and execute command lines from standard input

answered Feb 15 '17 at 06:30

Inian

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My question was more about how pipes work, but thanks for your answer nonetheless. – Kenny Worden Feb 15 '17 at 18:09

Pipe printf to ls in Bash?

2 Answers2