what's the exact differences between jsch ChannelExec and ChannelShell?

The shell and exec channels are quite similar - both execute commands with the remote shell (at least, conceptually - the server might be configured to treat them differently, of course). RFC 4254 groups them in the section "Interactive Sessions", and they both (as well as subsystem, see below) use the channel type "session" in the SSH protocol.

There is one important difference:

• For ChannelShell, the input stream provides both the commands and input to these commands. This is like using an interactive shell on your local computer. (And it is normally used just for that: interactive use.)

• For ChannelExec, the commands are given with setCommand() before connect(), and the input stream will be forwarded to these commands as input. (Most often, you will have only one command, but you can provide multiple ones using the normal shell separators &, &&, |, ||, ;, newline, and compound commands.) This is like executing a shell script on your local computer. (Of course, if one of the commands itself is an interactive shell, this will behave like a ChannelShell.)

• There is a third similar one, ChannelSubsystem, which executes a subsystem of the ssh server - here the server's configuration decides what to do, not the remote user's shell. (The most often used subsystem is sftp, but for this JSch provides a specialized channel, which understands the protocol.)

Note that what I call "input stream" here is the stream of data in the channel from the local to the remote host – that can be actually be done by passing a Java InputStream to the setInputStream method, or by getting a Java OutputStream from the getOutputStream method and writing to it.