Transparent remote command execution with sshfs
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I use SSHFS frequently so I can work on remote servers using local applications. The access times are generally acceptable on a modern broadband connection for basic text editing with Vim, but I/O heavy commands like git or grep with multiple files are insufferably slow. To resolve this, I wrote sshfsexec which transparently executes commands on remote systems that host locally mounted SSHFS volumes.

Basic Configuration and Usage

Copy sshfsexec to a folder defined in the PATH environment variable, and make sure the file is executable. The folder that sshfsexec is copied to should not contain any executable that shares the base-name of commands to be run on remote systems for reasons that should become clear later, so it is best to create a new folder specifically for sshfsexec and adjust the PATH environment variable accordingly.

$ mkdir ~/bin/sshfsexec
$ cp ~/bin/sshfsexec
$ chmod +x ~/bin/sshfsexec/
$ export PATH="$HOME/bin/sshfsexec:$PATH"

When adjusting the PATH environment variable, the login profile should also be edited (generally ~/.profile) so PATH is setup automatically upon login. Once sshfsexec has been copied into a folder in PATH, create symlinks from sshfsexec for the commands that should be executed transparently on the remote systems. In the example below, git will be executed on remote systems when interacting with SSHFS volumes.

$ cd ~/bin/sshfsexec
$ ln -s git
$ hash -r                     # Clears the executable path cache in Bash.

Any time git is launched inside of the SSHFS volume, it will be executed on the remote system transparently:

~$ sshfs
~$ cd
mydwm.git$ git log | head
commit 8f80c5343a8b430b28d18a6902ed2fbb05a6c90a
Author: Eric Pruitt
Date:   Wed Dec 26 19:53:45 2012 -0600

    Folder paths normalized in dwmstatus

    - Folder paths may now end with slashes, and repeated slashes are
      normalized to a single slash.

commit bfd709906a5a5cc0f3788e342bbc47d7e6aa5a92

That is not a particularly convincing example. Let us compare git --version executed inside the SSHFS volume and the local home directory:

mydwm.git$ git --version
git version 1.7.1
Connection to closed.
mydwm.git$ cd
~$ git --version
git version

Whenever a command should be executed locally instead of remotely, sshfsexec will traverse the folders in PATH looking for an executable with the same name that was used to invoke sshfsexec. Once an executable is found, it is launched with the arguments originally passed to sshfsexec.

Tips, Tricks and Ideas

Improving Performance

With the default SSH client configuration, a new SSH connection to the remote server must be created each time a command is executed. Using the configuration options ControlMaster and ControlPath to setup shared SSH connections will reduce the latency of command invocation on remote servers. Here is a comparison of command execution time with and without SSH connection sharing:

# SSH connection sharing not enabled
codevat$ time grep &> /dev/null
real    0m0.927s

# Unmounting remote server and enabling SSH connection sharing
codevat$ cd
~$ fusermount -u codevat/
~$ vi ~/.ssh/config
~$ sshfs codevat/

# SSH connection sharing enabled
~$ cd codevat/
codevat$ time grep &> /dev/null
real    0m0.135s

To enable SSH connection sharing, add the following lines to the local SSH configuration at ~/.ssh/config:

ControlMaster auto
ControlPath /tmp/ssh_mux_%h_%p_%r

Next, unmount and remount the SSHFS volume to begin using connection sharing. Check out the documentation in ssh_config(5) for more information on SSH connection sharing.

SSH Connection Messages

SSH will display a message like "Connection to $HOSTNAME closed." or "Shared connection to $HOSTNAME closed." whenever an SSH session with a pseudo-terminal terminates. Although this message can be disabled with Loglevel=quiet, doing so also disables at least one critical security message as noted by a commenter in OpenSSH bug #1273. I found these messages annoying, but I was unwilling to accept the possibility of suppressing important notices. I patched my OpenSSH client binary, changing the first character of each format string to a null byte, to get rid of the messages. I used xxd and vim the first time I patched the binary, but provided the format strings in the targeted version of OpenSSH do not differ from mine, the following perl substitution should work just as well:

$ ssh -V
OpenSSH_5.5p1 Debian-6+squeeze2, OpenSSL 0.9.8o 01 Jun 2010
$ strings /usr/bin/ssh | egrep -i '(Shared )?connection to \S+ closed\.'
Connection to %.64s closed.
Shared connection to %s closed.
$ cp /usr/bin/ssh ~/bin
$ perl -pi -e 's/((Shared )?connection to \S+ closed)\./\0\1/ig' ~/bin/ssh

Usage Ideas

Here are some of the most frequently used programs I have symlinked to sshfsexec:

crontab  egrep  fgrep  find  git  grep  last  mysql  php  service  w  wget

Advanced Configuration

Configuration is managed with a Python script read from ~/.sshfsexec.conf or the path defined by the environment variable SSHFSEXEC_CONFIG. This script will be executed twice, once before the command arguments are translated and once after. On the first pass, pre_process_config is True and False on the second pass. During the first execution, remoteargs will be unpopulated, and sshlogin will not yet be set if sshfsexec was run outside of an SSHFS volume. A sample configuration script named "" is included with the source code to sshfsexec.

When the configuration script is executed, it is executed inside of sshfsexec's main function with unrestricted access to the code, but a list of the most relevant variables follows below.


pre_process_config: Variable indicating whether or not the configuration file is being executed before or after the command arguments have been translated to remote paths.

command: Base-name of the command being executed (os.path.basename(sys.argv[0])) when the sshfsexec is launched.

coerce_remote_execution: Determines whether or not referencing a path within an SSHFS volume will coerce a command to be executed remotely instead of locally. When this option is set, if a command's arguments reference paths that are within an SSHFS volume, one's current working directory need not be inside the mount for transparent usage. In the following example, git would be executed remotely with coerce_remote_execution set:

~$ git clone ~/ ~/
Initialized empty Git repository in /home/git/repositories/clone.git/.git/

This option is disabled by default and must be set before the command arguments are parsed / while pre_process_config is True.

preserve_isatty: A PTY will normally only be allocated on the remote server if the local stdin and stdout are pseudo-terminals. This causes problems with things like grep --color and ls --color=auto, both of which check the value of isatty on stdout to determine whether or not colors should be rendered. Setting preserve_isatty will result in some kludgey stuff being done to make sure the isatty results for stdin, stdout and stderr on the remote process are the same as for the local sshfsexec process.

In its current implemenation, this option is buggy: anything launched on the remote server that reads from stdin will hang indefinitely even when no more data is available and the local pipe has been closed.

Command Execution Variables

environment: A dictionary containing the environment variables that will be passed to the subprocess. Note that for commands run via SSH, the environment variables are set locally before SSH is launched. This means that environment variables are subject to the configuration settings on the remote host, specifically PermitUserEnvironment.

envpassthrough: This dictionary is used to configure environment variables on the remote server. Normally, only environment variables permitted by the remote server's sshd configuration will be made available to the remote process, but with envpassthrough, the environment variables will be declared on the remote server before executing the commands. For example, if envpassthrough = { "EDITOR": "vim" }, the remote invocation of git commit will be as follows: EDITOR=vim git commit. Please note that the definition of these environment variables is likely to be visible to all users on both the remote server and local host, so envpassthrough should not be used for sensitive information.

stdin_is_pipe: Boolean indicating whether or not stdin is a pipe.

originalargs: Iterable containing the untranslated arguments passed to the command (sys.argv[1:]).

sshlogin: Remote SSH system where a command will be executed. This will be in the form of user@hostname or simply hostname depending on the arguments used for sshfs and the defined SSH options.

cwdtranslation: When a command is launched inside a directory within an SSHFS mount, cwdtranslation is a tuple that contains the remote login ("user@hostname" or "hostname"), the remote directory that corresponds to the current working directory, and the SSHFS local mount point for SSHFS volume. If a command is launched outside of an SSHFS folder, this variable will be None.

remoteargs: Arguments that will be used to execute the command on the remote system. If any of the original arguments were paths on the remote system, the path in remoteargs will be the path as it exists on the remote server instead of how it was referenced within the SSHFS mount.

To Do

  • Allow commands with both local and remote path arguments to be executed on both systems. For example rm here.jpg ~/mounts/sshfs/there.jpg would run rm here.jpg locally and ssh user@remote 'rm there.jpg'.
  • Fix hanging issue with preserve_isatty.