Why does ch_evalraw() evaluate to my message instead of the response?

Question

I am trying to learn about jobs and channels on Vim (on Windows), and I am using the following sequence as an example:

let job = job_start("cmd")
let channel = job_getchannel(job)
let response = ch_evalraw(channel, "dir\n")

Here echo response yields dir instead of what I expect, the first line of output from dir (Volume in drive C is OS). Why is this the case, and how can I modify this to get what I expect?

Answer 1

The reason why you're seeing the command you just ran coming back in the channel is because Windows' cmd.exe will by default run with “echo” on, which means it will print back the command to be executed before executing it and sending back the output produced by it.

You can use job_start("cmd /q") to have it disable “echo” at startup, which should solve the problem and have your read from the channel return the first line produced by the dir command. See documentation for cmd.exe at Microsoft's website.

Another alternative is using, for example, bash.exe from a distribution such as Git Bash, which will not produce the “echo” you've seen with cmd.

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Note that a read from the channel will only produce the first line of output from dir and further channel reads don't seem to send the next lines of output from that command. I haven't really used Vim jobs and channels extensively, so I'm not really sure what the issue there is. You might want to ask a separate question about that if you'd like to explore that further...

If all you want is to capture the output of an external command, a much easier approach is to use the system() or systemlist() functions, which will take care of all the details for you and simply return a string or list. For example, to put the output of dir under the cursor in the current buffer, all you need is:

:put =systemlist('dir')