Shell – Can a command be executed without a shell

commandshellssh

I am developing a binary executable program command to execute programs and return the output into a text field.

The parameters to the command involve the command itself as it would be typed on the command line, and the directory. So the routine that executes the operation first switches to the directory, then executes the command.

For example if I want to execute the command some.cmd in the directory /home/user the parameters are command = 'some.cmd' and directory = '/home/user'.

What I have found is that some.cmd does not work but if I change command to /home/user/some.cmd the command works. However the command ls -l works. I also notice that the cd command is not recognized. If I run it remotely via ssh such as setting command to ssh user@localhost 'cd /home/user && ./some.cmd' it works.

It seems that some settings which are present when the command is executed in a shell are not present when it is run directly, but doing it via ssh seems to create the settings for it work.

Is there some explanation for this?

UPDATE:
After some enquiries I got to learn that the API used for executing the commands were not being executed in the shell, or were not executed with the normal environment available from the console. After executing the commands with the /bin/sh -c "cd ..." option the problem is no more. This is the environment doing ssh user@localhost 'command ...' gave me.I am not so sure of the technical details, but apparently the existence of the environment available when you execute in your normal shell is not always available to commands executed directly by the OS.

Best Answer

When you try to execute a file, the system has to know how to find the file. That's why it works if you specify the full path to it. The shell also has a PATH environment variable that stores a list of directories to look in to find an executable. That's why you don't have to specify the full path for ls.

Related Solutions

SSH Login – Can Commands Be Executed with Nologin User

Setting /sbin/nologin as the user's shell (or /bin/false or /bin/true, which are almost equivalent) forbids the user from logging in to run any command whatsoever. SSH always invokes the user's login shell to run commands, so you need to set the login shell to one that is able to run some commands.

There are several restricted shells that allow users to run only a few commands. For example rssh and scponly are both such shells that allow the user to run a few predefined commands (such as scp, sftp-server, rsync, …). See also Restrict user access in linux and Do you need a shell for SCP?

Shell – Why some commands don’t load user environment when executed with ssh? (while other do)

ssh vagrant@127.0.0.1 -p 2222 echo $PATH

Your local system's PATH will be printed here. You should quote the variable to prevent it from being expanded by the local shell:

ssh vagrant@127.0.0.1 -p 2222 'echo $PATH'

The same applies to:

ssh vagrant@127.0.0.1 -p 2222 echo $(which brew)

As for:

ssh vagrant@127.0.0.1 -p 2222 echo test ; which brew

which brew is run after the SSH command ends, by your local shell. That's why this didn't work:

ssh vagrant@127.0.0.1 -p 2222 /bin/sh -c "echo\ test\ \;\ which\ brew"

Side note: you don't need to do all that escaping.

The problem here might be that you're adding brew to your PATH in .profile (or .bash_profile or some similar file), which is run when a login shell is started. SSH starts a login shell only when it is run interactively, not when it's told to run a command. From man ssh:

 If command is specified, it is executed on the remote host instead of a login shell

You could try:

ssh vagrant@127.0.0.1 -p 2222 bash -lc brew

Best Answer

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SSH Login – Can Commands Be Executed with Nologin User

Shell – Why some commands don’t load user environment when executed with ssh? (while other do)

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