Pipe Command – How to Pipe Stdout on One Server to Stdin on Another Server

pipestdinstdout

stdout on one CentOS server needs to be piped to stdin on another CentOS server. Is this possible?

Update

ScottPack, MikeyB and jofel all have valid answers. I awarded the answer to Scott because, even though my question didn't specify security as a requirement, it's always nice to be safe. However, the other two fellows' suggestions will also work.

Best Answer

This is an unabashed yes.

When one uses ssh to execute a command on a remote server it performs some kind of fancy internal input/output redirection. In fact, I find this to be one of the subtly nicer features of OpenSSH. Specifically, if you use ssh to execute an arbitrary command on a remote system, then ssh will map STDIN and STDOUT to that of the command being executed.

For the purposes of an example, let's assume you want to create a backup tarball, but don't want to, or can't, store it locally. Let's have a gander at this syntax:

$ tar -cf - /path/to/backup/dir | ssh remotehost "cat - > backupfile.tar"

We're creating a tarball, and writing it to STDOUT, normal stuff. Since we're using ssh to execute a remote command, STDIN gets mapped to the STDIN of cat. Which we then redirect to a file.

Related Solutions

What is Meant by Connecting STDOUT and STDIN?

Standard input and standard output are not commands.

Imagine commands as machines in a factory with an assembly line. Most machines are designed to have one conveyor belt to feed data in and one conveyor belt to feed data out; they are the standard input and the standard output respectively. The standard error is an opening on the side of the machine where it can eject rejects.

+-------+     +------------------+       +------------------+     +------+
| input |     |    machine A     |       |    machine B     |     |output|
| reser |=====|<stdin     stdout>|=======|<stdin     stdout>|=====|bucket|
| ‑voir |  →  |      stderr      |   →   |      stderr      |  →  |      |
+-------+     +------------------+       +------------------+     +------+
                      ||                          ||

The diagram above shows a conveyor belt that goes through two machines. The data comes from the input reservoir on the left, is fed to machine A, then the output is conveyed further to machine B (for which it is input), and machine B's output is deposited in the output bucket on the right.

In unix terms, this is called a pipeline. The metaphor is that of plumbing: a pipe connects machine A to machine B. The shell syntax for the pipeline above is

<input-file.txt commandA | commandB >output-file.txt

The < redirection symbol tells the shell to connect commandA's standard input to the file input-file.txt before launching commandA. (You can put the redirection before or after the command name.) The > redirection symbol tells the shell to connect commandB's standard output to output-file.txt. The pipe ("|") symbol in the middle tells the shell to connect commandA's standard output to commandB's standard input before launching them.

Commands can have more than one input and more than one output, but that's material for another day.

Shell – How to pipe stdout to another program

Just pipe through a while loop:

git diff --name-only develop | grep coffee$ | while IFS= read -r file; do
    ./node_modules/.bin/coffeelint "$file"
done

Update

Best Answer

Related Solutions

What is Meant by Connecting STDOUT and STDIN?

Shell – How to pipe stdout to another program

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