Using elixir variable as System.cmd input

Questions / Help
1

Hi there,
I was wondering if there is a way to redirect input to a system command from an elixir variable instead that from a file.

In other words I’d like to invoke a command like this:

wc -l < myfile

using Elixir’s System.cmd:

my_stream = """
a
b
c
"""

"wc"
|> System.find_executable
|> System.cmd(["-l", ...])

After some search I think that the only way could be using in some way IO.Stream, but I can’t figure out how.
The variable my_stream contains the stream to be provided to wc -l.

Any suggestion?

Thank you
Carlo

1 Like
2

That < syntax in shell is just a way to pipe the contents of a file onto the STDIN of the command. Interestingly, wc will also take a filename as a regular argument. So for either of those to work you would need to write your string to a file and use that (although I’m not sure the < would work anyway, because I believe it’s a shell feature).

But, it’s fairly simple to write to the STDIN of a command from Elixir without using any special shell syntax, just that you need to use the underlying Port module directly (which is what System.cmd is using under the hood).

All that having been said, if you’re really just trying to count the number of lines in the string:

my_stream |> String.split("\n") |> Enum.count()
1 Like
3

If you want pipes, you can use them.
The documentation for System.cmd/2 mentions

Shell commands

If you desire to execute a trusted command inside a shell, with pipes, redirecting and so on, please check :os.cmd/1.

so you can do

iex(11)> :os.cmd('wc -l < foo.txt') |> :binary.list_to_bin()
"5\n"

If you want to read from a variable, you interpolate it, or set an environment variable

iex(14)> System.put_env("foo", "bar baz")                     
:ok
iex(17)> :os.cmd('echo ${foo}') |> :binary.list_to_bin()        
"bar baz\n"
iex(18)> :os.cmd('echo ${foo} | wc') |> :binary.list_to_bin()   
"      1       2       8\n"
4

Nice, thanks for sharing that. Very cool.

1 Like
5

Thank you.

My real purpose is not to count the line of a file, it was a simplification of what I am trying to, launching an external tool via System.cmd and providing an input from STDIN. I am going to give a look to the port module, thank you

1 Like
6

It seems a really good solution, thank you

1 Like