You're in Cape Town, but your data is in California. You're using a Sun system in your office, but your bleeding-edge Mozilla browser with the very cool theme and all of your favorite bookmarks is on your Linux box at home. How can you get bits from there to here without being there?
You’re in Cape Town, but your data is in California. You’re using a Sun system in your office, but your bleeding-edge Mozilla browser with the very cool theme and all of your favorite bookmarks is on your Linux box at home. How can you get bits from there to here without being there?
Linux has more remote access solutions than you might realize. You’ve probably heard of scp and ssh. But did you know that you can run an X application securely through an ssh tunnel? Or that ssh can run almost any command line on the remote system — not just a simple command? Indeed, both (and more) are possible, so let’s dig in and learn how.
If you aren’t familiar with ssh, see the introduction to OpenSSH in the December 2000 issue of Linux Magazine, available online at http://www.linux-mag.com/2000-12/openssh_01.html.
scp Helpers: Two Shells
The secure file-copy command, scp, has a syntax like the Linux cp command: the first arguments are the files to copy and the final argument is the destination. Unlike cp, scp lets you copy files to and from a remote host. If you want to refer to a file on a remote host, simply prepend a hostname (and a username if it’s different than your local username) and a colon (:) to the file path and name.
For instance, to copy both the file .cshrc from your home directory on host foo.bar and the file prog from zoe‘s bin subdirectory on host baz into the current directory (.) on your current host, use the command:
% scp foo.bar:.cshrc zoe@baz:bin/prog .
Enter your ssh password or passphrase if you’re prompted for it. (If you don’t want to repeatedly type your password or passphrase, check out ssh-agent and ssh-add. The sidebar “SSH Agent” has more details and setup tips.)
How can you copy all of the files from your local bin directory to zoe‘s bin directory on baz? A wildcard will do it:
(If you need a refresher on relative pathnames like bin/*, see “What’s in a Pathname?” in the October 2002 issue, available online at http://www.linux-mag.com/2002-10/power_01.html.)
Now, let’s try to turn the previous example around and copy all files from zoe‘s remote bin to your local bin:
% scp zoe@baz:bin/* bin
zoe@baz:bin/*: No match.
Unfortunately, your local shell won’t run this scp command because the wildcard doesn’t match.
What’s wrong? When you type a wildcard like * or ? on a command line, the shell (not the application program, scp) tries to expand the wildcard into matching pathnames on the local host where the shell is running. (The shell doesn’t know anything about scp or remote hosts. It just knows how to expand wildcards into pathnames.) Because there’s (probably!) no local subdirectory named zoe@baz:bin, the shell prints No match and aborts without running the application (scp).
(By the way, if you use bash, the scp command will probably succeed — but only by luck. A Bourne-type shell starts scp and passes the unexpanded argument to it. But there’s always a chance that your argument will expand into a pathname on the local host, so it’s always safer to tell the local shell, even bash, “Don’t try to interpret this wildcard!”)
You can prevent processing of wildcards by the local shell by, escaping (or quoting) the wildcard character(s) that you don’t want interpreted like this:
This scp command works. Why? Here, the local shell sees the backslash, removes it and passes the result (zoe@baz:bin/*) to scp. scp parses the argument, finds the username and hostname, and passes the rest of the command line to a remote shell running on baz. The remote shell tries to expand its command line argument, which is bin/*. Since the remote scp shell’s current directory is the home directory on that host, the wildcard pathname expands into all relative pathnames starting with bin/, which is bin/afile, bin/bfile, and so on. Finally, the remote shell gives those arguments to the remote scp process, which arranges to copy those files back to your local scp process, which writes them to the destination.
Why is this Shell Stuff Important?
Once you understand that there are two shells — one local shell and one remote shell — involved with scp (and, as we’ll soon see, with ssh), you can use the shells’ interpretation and execution powers to do much more than just copying a few files or starting a remote program.
Let’s consider an important example: copying a directory tree. The scp -r option copies all files and links from a directory and all of its subdirectories. But, like cp -r, the scp -r command creates plain files — not links — at the destination. It also copies files one-by-one, which can be tedious and a bit slow if the directory tree is big.
The shell (and other programs) can solve this problem — and a lot of others! But first we need some more background.
The ssh Data “Pipes”
scp and ssh open an encrypted connection between your local host and each remote host, and use the connection (or connections) to transfer data in both directions. The arguments to scp are pathnames, while the arguments to ssh are complete command lines.
Here’s a trivial example: assuming you have a home directory on the remote host named bar, the following command lists the contents of that directory:
% ssh bar ls -F
How does it work? The local ssh got three command-line arguments: bar, ls, and -F. The first argument is the remote hostname and the rest of the arguments represent the command line for the remote shell. The remote ssh process runs ls -F, encrypts the standard output of that ls process, and routes the encrypted output to the originating (local) ssh process. The local ssh process decrypts the informations and prints it to its own standard output in your terminal window.
How can you save that listing into a file named bar-ls? Just use the shell’s redirection operator > like this:
% ssh bar ls -F > bar-ls
% cat bar-ls
Redirection is another instance where it’s important to understand how the shell works. Before the shell runs the local application (ssh) it first arranges for the standard output of that process to go to the file bar-ls. Then the local shell runs the program. So, the file will be created on the local host, by the local ssh and the local shell!
What if you wanted to run a remote process and redirect its output to a file on the remote host? Tell the remote shell to do the redirection. How do you do that? By quoting the > operator. That prevents the local shell from interpreting the redirection operator. It’s probably simplest if you quote the entire remote command line, like this:
% ssh bar ‘ls F > bar-ls’
In this case, the local shell strips off the quoting characters, ignoring the special meaning of whatever’s between them, and passes that string (ls -F > bar-ls) to the local ssh. The local ssh passes that same string to the remote ssh, which passes it to the remote shell. The remote shell parses the command string and executes it. Because the > isn’t quoted by the time it reaches the remote host, it’s interpreted as the redirection operator. So the file bar-ls is created in the remote shell’s current directory, which, for ssh, is the home directory of the remote account.
Got it? (If not, please re-read the previous section. The concepts are important.) Otherwise, great! Now the fun starts.
Example: Copying Files via ssh
Let’s get back to our earlier problem: how to copy a directory tree, working around scp‘s weaknesses.
The tar utility is made for reading a directory tree and packing it into an archive file — or into a stream of bits. The tar flag f tells it to write or read the archive from a file. If that filename is -, tar writes to its standard output or reads from its standard input.
Let’s start by running tar on the remote host and redirecting its output to a file on the local host. This is similar to the ls -F command we ran earlier. (Here we’ll quote the remote command line, but for no reason except to make it stand out more clearly. There are no special characters that the local shell shouldn’t interpret.) Once we have our local bar-bin.tar file, we’ll run a local tar to unpack the archive.
% ssh bar ‘tar cf – bin’ > bar-bin.tar
% tar xvf bar-bin.tar
…verbose output of tar…
Ultimately, we don’t need the intermediate file bar-bin.tar because tar can also read an archive stream from standard input. How do you feed the standard output of a process to the standard input of another process? Use a pipe, the shell’s | operator, like this:
% ssh bar ‘tar cf – bin’ | tar xvf -
…verbose output of tar…
Again, you might ask yourself, “Which process is running where?” The answer: the remote ssh is running tar cf – bin and feeding its output (the archive stream) to the local ssh. The standard output of the local ssh is piped to tar xvf -, which is running on the local host, reading the archive stream from its standard input, and (in this case) writing it to the local filesystem. After this command runs, a copy of the entire remote bin directory tree appears on the local host.
By default, tar doesn’t compress its archive. This means our file copy takes more time and bandwidth than it otherwise needs to. So we’ll use bzip2 (a cousin of gzip) to compress the archive stream before it leaves the remote host and then use bzcat to expand it on the local host and pass it to our local tar. The command line to use is shown in Listing One.
Listing One: Compressing an ssh data stream
% ssh bar ‘tar cf – bin | bzip2′ | bzcat | tar xvf -
…verbose output of tar…
The remote ssh starts a shell which runs the command tar cf – bin | bzip2. The last two | characters aren’t quoted, so the local shell makes two pipes: one feeds the output of the local ssh to a local bzcat process, which expands the compressed archive stream. Using the next pipe, the output of the local bzcat process is piped to the local tar process, which reads and stores the expanded archive bits.
Let’s wrap up with a few more examples:
1% scp “bar:lib/[a-c]*” .
2% ssh bar cat bigf | less
3% ssh bar gzip -c bigf | zcat | less
4% scp bar:bigf bigf.bar
5% ssh bar gzip -c bigf | zcat > bigf.bar
6% gzip -c hugef | ssh bar ‘zcat > hugef.rem’
7% ssh bar lpr -Pinkjet_jpeg < Kristy.jpg
- Command 1 copies all files whose names start with a, b, and c from the lib directory on bar to the local current directory.
- Command 2 runs cat bigf on bar and pipes it to a local less pager, which displays the file in your terminal. Command 3 is like command 2, but it compresses the data on the remote host and uncompresses it locally before displaying it.
- Command 4 copies bigf from bar to a local file named bigf.bar; Command 5 does the same things but compresses the file to save bandwidth.
- Command 6 copies the local file hugef to a file named hugef.rem on bar. The quotes mean that the > redirection is done on bar.
- Command 7 uses the shell’s < redirection operator to feed a JPEG photo to the standard input of your local ssh. The remote ssh starts lpr -Pinkjet_jpeg which, because it doesn’t have any other arguments, reads standard input (which has the photo to print).
Saving keystrokes with ssh-agent
The job of the ssh-agent and ssh-add programs is to collect your passphrase once — typically, as you start your login session — and then let you use ssh and scp commands without entering your passphrase again. This makes working with remote systems almost as easy as working on your local host. Learning how it works will help you set it up, so let’s have a look.
ssh-agent makes a socket in a subdirectory of /tmp and starts a detached process that negotiates with later ssh and scp commands. It also outputs shell commands that set two environment variables. Because of that, you need to run ssh-agent using command substitution (backquotes) and the shell’s eval command to read that output. Finally, ssh-add adds identities to the agent.
% eval `ssh-agent`
Agent pid 272
Enter passphrase for identity ‘jp@laptop’:
You can see the environment variables set by ssh-agent –SSH_AUTH_SOCK and SSH_AGENT_PID — by searching the output of env, which prints all current settings:
% env | grep SSH
% ls -l /tmp/ssh-natVG242/
srwx–x–x 1 jpeek … agent.242
The two environment variables, SSH_AUTH_SOCK and SSH_AGENT_PID, are propagated to all subprocesses of this shell. So you’ll want to start the agent process as high as possible in your process tree, ideally before you start your window system. (You can run ssh-add after your window system is running.)
Later ssh or scp jobs will check their environment to locate the agent. If the two environment variables aren’t set (weren’t passed from the environment of the shell that started them), ssh and scp will prompt for a passphrase.
If you don’t have an easy way to start the agent before your window system, start it from a shell prompt in one window and save its output (the environment-setting commands) to a temporary setup file. Next, from all windows where you want to use ssh, read the commands from the setup file into the shell using the command source for C-like shells or . (a dot) for Bourne shells. (Using the option -c for C shells or -s for Bourne shells makes sure ssh-agent uses the right syntax.) So, on the first shell, you’d do:
% ssh-agent -c > ~/ssh-agent-setup
% source ~/ssh-agent-setup
Agent pid 272
Then, in other shells, simply repeat the source command to set the variables in those shells.
X Tunnels Through ssh
We’ve seen that ssh can do more than file transfers. It can run any program on the remote host, with one class of exceptions: you can’t run interactive, full-screen programs like less or vi with a command like ssh baz vi somefile because the remote ssh process isn’t running in a tty or pty.
However, you can do an interactive login to a remote host, with a simple command like ssh baz. This starts an interactive shell, in a pty, on the remote host.
ssh can also run an X application on a remote host, with the display appearing on your local X host. Better yet, ssh can tunnel the otherwise insecure X protocol (meaning prying eyes can see the raw X traffic) through a secure, encrypted connection and handle all the messy details of authentication, too. This is called X forwarding.
Using X forwarding is simple: just run the X application like any other ssh command line. Type ssh, the hostname, and the name and arguments of the X application. You may need to give an absolute pathname (like /usr/X11R6/bin/xterm) if the X application is in a directory that the remote shell doesn’t know about. (Hint: because the remote ssh starts a non-login shell, you can try setting the remote shell’s search path in a non-login setup file like .bashrc or .tcshrc.)
Try this first simple X command to display the remote host’s load average graph on your local display. (Choose a remote host that has X installed.)
If you get the error xload: command not found, try /usr/X11R6/bin/xload instead. For other ssh problems, add the ssh option -v to get verbose output. If that still doesn’t help, see a good ssh reference — like O’Reilly’s book SSH, The Secure Shell. If all’s well, though, a little xload window should pop up within a few seconds. Use CTRL-C to kill ssh. The xload window should close a moment later.
Here are a few more commands to try. These use the shell’s & (ampersand) operator to run the ssh process in the background. This lets you open multiple remote processes from the same local terminal. When you exit the remote process, the local ssh process exits too. (Job control commands, like kill %3 to kill the third job, should work too, though they may not give the remote application time to quit cleanly.)
8% ssh baz /usr/local/bin/mozilla &
9% ssh baz xterm &
10% ssh baz xterm -e vi afile &
11% xterm -e ssh baz &
- Command 8 starts the Mozilla web browser from host baz, with all the local customizations you’ve made there. (Mozilla’s release notes mention that Shockwave plug-ins before Version 6 may not work on remote X displays.)
- Command 9 starts xterm running on the remote host.
- Command 10 starts a remote vi running inside an xterm.
Both commands 9 and 10 send all of the graphical traffic across the network. It’s probably more efficient to start the xterm locally and have it open an interactive shell, as in command number 11, where you can then type the vi command.
Next month’s column will cover ways to automate file transfers with ftp, rsync and friends. See you then!
Do you need to go to some directory to run a command or two, then come back to your original directory? In bash and some other shells, the command cd - changes to your previous directory. Another cd - brings you back.
A similar trick that works in all shells is a subshell. Because a subprocess can’t change its parent’s environment, the subshell can cd but, when it exits, its parent shell has the same current directory. Here’s an example:
$ (cd /longpath; ./configure; make)
…configure and make run…
Here’s another example: this command creates an alias that cds to another directory and runs commands there without changing the current directory in your shell:
alias setistat ‘(cd /tmp/seti; ps `cat pid.sah`; grep “prog=” state.sah)’
Jerry Peek is a freelance writer and instructor who has used Unix and Linux for over 20 years. He’s happy to hear from readers at email@example.com.