I want to execute a program through my Elixir code. What is the method to call a shell command to a given string? Is there anything which isn't platform specific?
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7 Answers
Here is how you execute a simple shell command without arguments:
System.cmd("whoami", [])
# => {"lukas\n", 0}
Checkout the documentation about System for more information.
Comments
System.cmd/3 seems to accept the arguments to the command as a list and is not happy when you try to sneak in arguments in the command name.
For example:
System.cmd("ls", ["-al"]) #works, while
System.cmd("ls -al", []) #does not.
What in fact happens underneath is System.cmd/3 calls :os.find_executable/1 with its first argument, which works just fine for something like ls but returns false for ls -al for example.
The Erlang call expects a char list instead of a binary, so you need something like the following:
"find /tmp -type f -size -200M |xargs rm -f" |> String.to_char_list |> :os.cmd
1 Comment
bortzmeyer
This behaviour is goof for security; if the string passed to System.cmd comes from an unsafe source, not using a shell to parse it closes some attack vectors.
You can have a look in the Erlang os Module. E.g. cmd(Command) -> string() should be what you are looking for.
2 Comments
José Valim
Or
System.cmd/1 from Erlang that works with Elixir strings. :)bortzmeyer
There is another important difference between the two: stackoverflow.com/questions/22594988/…
The "devinus/sh" library is another interesting approach to run shell commands.
1 Comment
fregas
Just FYI, this hasn't been updated in a couple years
I cannot link directly to the relevant documentation but it's here under the System module
cmd(command) (function) #
Specs:
cmd(char_list) :: char_list
cmd(binary) :: binary
Execute a system command.
Executes command in a command shell of the target OS, captures the standard output of the command and returns the result as a binary.
If command is a char list, a char list is returned. Returns a binary otherwise.
answered Mar 24, 2014 at 12:12
Onorio Catenacci
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Comments
One could also use erlang's :os module like so:
iex(3)> :os.cmd('time')
'\nreal\t0m0.001s\nuser\t0m0.000s\nsys\t0m0.000s\n'
Beware that you'll have to handle erlang binaries when processing the result :os.cmd('time') |> to_string() for example
1 Comment
bortzmeyer
Also,
:os.cmd is quite different from System.cmd and more dangerous: it passes the string directly to the shell, which will interpret special characters such as *. Never call :os.cmd with variables provided from the outside. System.cmd does not use the shell and is therefore safer.If I have the following c program in the file a.c:
#include <stdio.h>
#include <stdlib.h>
int main(int arc, char** argv)
{
printf("%s\n",argv[1]);
printf("%s\n",argv[2]);
int num1 = atoi(argv[1]);
int num2 = atoi(argv[2]);
return num1*num2;
}
and compile the program to the file a:
~/c_programs$ gcc a.c -o a
then I can do:
~/c_programs$ ./a 3 5
3
5
I can get the return value of main() like this:
~/c_programs$ echo $?
15
Similarly, in iex I can do this:
iex(2)> {output, status} = System.cmd("./a", ["3", "5"])
{"3\n5\n", 15}
iex(3)> status
15
The second element of the tuple returned by System.cmd() is the return value of the main() function.
