CODEWITHSUNDEEP
linuxbashshellcrystal-lang
Sod Almighty
Sod Almighty

Reputation: 1786

How to execute a shell script in Crystal while capturing output?

I want to execute a shell script while handling stdout and stderr output. Currently I execute commands using Process.run, with shell=false and three pipes for stdin, stdout and stderr. I spawn fibers to read from stdout and stderr and log (or otherwise process) the output. This works pretty well for individual commands, but fails horribly for scripts.

I could simply set shell=true when calling Process.run, but looking at the Crystal source it seems that merely prepends "sh" to the commandline. I've tried prepending "bash" and it didn't help.

Things like redirection (>file) and pipes (e.g. curl something | bash) don't seem to work with Process.run

For example, to download a shell script and execute it, I tried:

cmd = %{bash -c "curl http://dist.crystal-lang.org/apt/setup.sh" | bash}

Process.run(cmd, ...)

The initial bash was added in the hope that it would enable the pipe operator. It doesn't seem to help. I also tried executing each command separately:

script.split("\n").reject(/^#/, "").each { Process.run(...) }

But of course, that still fails when a command uses redirection or pipes. For example, the command echo "deb http://dist.crystal-lang.org/apt crystal main" >/etc/apt/sources.list.d/crystal.list simply outputs:

"deb http://dist.crystal-lang.org/apt crystal main" >/etc/apt/sources.list.d/crystal.list`

It might work if I used the `` backticks method of execution instead; but then I wouldn't be able to capture the output in real time.

Upvotes: 7

Views: 4351

Answers (3)

Oliver
Oliver

Reputation: 21

or if you want to call a shell script and get the output i just tried with crystal 0.23.1 and it's work ! 

def screen
    output = IO::Memory.new
     Process.run("bash", args: {"lib/bash_scripts/installation.sh"}, output: output)
     output.close
    output.to_s
end

Upvotes: 2

Julien Portalier
Julien Portalier

Reputation: 2999

The problem is a UNIX problem. The parent process must be capable to access the STDOUT of the child process. Using a pipe you must start a shell process that will run the whole command, including the | bash and not just curl $URL. In Crystal this is:

command = "curl http://dist.crystal-lang.org/apt/setup.sh | bash"
io = MemoryIO.new
Process.run(command, shell: true, output: io)
output = io.to_s

Or if you want to duplicate what Crystal does for you:

Process.run("sh", {"-c", command}, output: io)

Upvotes: 13

Anya Shenanigans
Anya Shenanigans

Reputation: 94749

I'm basing my understanding on reading the source code of the run.cr file. The behaviour is very similar to other languages in how it deals with commands and arguments.

Without shell=true, the default behaviour of Process.run is to use the command as the executable to run. This means that the string needs to be a program name, without any arguments, e.g. uname would be a valid name as there's a program on my system called uname in /usr/bin.

If you ever got behaviour of successfully using %{bash -c "echo hello world"} with shell=false, then something is wrong - the default behaviour should have been to try to run a program called bash -c "echo hello world", which is unlikely to exist on any system.

Once you pass in 'shell=true', then it does sh -c <command>, which will allow strings like echo hello world as a command to work; this will also allow redirections and pipelines to work.

The shell=true behaviour can generally be interpreted as doing the following:

cmd = "sh"
args = [] of String
args << "-c" << "curl http://dist.crystal-lang.org/apt/setup.sh | bash"
Process.run(cmd, args, …)

Note that I'm using an array of arguments here - without the array of arguments, you don't have any control over how the arguments are passed into the shell.

The reason why the first version, with or without shell=true doesn't work is because the pipeline is outside the -c, which is the command you're sending to bash.

Upvotes: 5

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