Speaking UNIX, Part 3
Do everything right from the command line
It's a wired, wired, wired world
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The UNIX® command line is a WYTIWYG interface -- that is,
What you type is what you get. UNIX provides hundreds, if not
thousands, of commands with which you can manipulate a large variety of
resources available in the kernel and user space. Need to monitor CPU
usage? Try top or ps. Need to remove all files
ending in .bak? Try rm *.bak. Want help with a new command?
Run man.
But what do you do when the resources you need reside on a remote system on your wide-area network (WAN) and on the global Internet? To quote The Hitchhiker's Guide to the Galaxy, "don't panic!". The UNIX command line readily downloads and uploads files, connects to remote computers, and interrogates the state of far-flung servers and networks. Grab your towel: It's time for a trip to extra-solar systems.
Work locally, transfer globally
In Part 1 and Part 2 of this series, you learned how much you can accomplish with the UNIX command line. With just a few keystrokes -- including a pipe (|) or redirection -- you can create an impromptu data processing machine more powerful than the sum of its parts.
While some of the resources you use daily are likely to be local -- that is, resident on your own workstation -- a significant and growing number of assets -- files, e-mail messages, and tools -- are likely to be stored at a distance (say, on a machine connected to your WAN or to the Internet). Web browsers provide almost universal access to such resources, with one caveat: Point-and-click can quickly become tedious, even onerous, especially if you must retrieve more than one handful of items. Moreover, if you want to script -- essentially, capture and replay -- repetitive or error-prone tasks, a windowing browser is a difficult ally.
Much like ls, cp, mail,
uptime, du manage, and
query local resources, UNIX has a suite of command-line tools
to access remote resources, too. This article introduces you to a few of
those tools, including a useful technique that both facilitates access to
remote systems and protects your authentication credentials. Specifically,
you'll learn wget, curl, and Secure shell
(ssh). The wget and curl tools
transfer files; using ssh, you can securely log in to remote
systems and transfer files quickly and easily.
The trouble with Telnet (and others)
If any of your systems run rsh (or its variants --
rcp, rexec, rlogin, or
Irdist) or telnet, disable and remove them and
the accompanying daemons immediately. In addition, if you don't permit
anonymous File Transfer Protocol (FTP), disable the FTP software, too.
Although rsh and telnet are longtime UNIX
stalwarts, attackers can leverage either utility to (easily) compromise
your system. You or your system administrator should halt and remove this
software wherever it's found running and replace the features of those
packages with ssh.
For privileged FTP access, use sftp. Replace
rdist with the more advanced rsync. Or, if you
must provide anonymous FTP (or downloads over HTTP), be sure to use
firewall hardware and software to isolate all publicly accessible
computers from sensitive internal servers.
But first, let's discuss the pesky problems that passwords present.
"You don't need no stinkin' passwords!"
Access to most computers and services is typically protected. In some cases, authenticating your identity (and hence your privilege to access the system) might require a complex challenge-response exchange, a Secure Sockets Layer (SSL) certificate, or even a biometric scan. Typically, however, a password suffices to gain access. Much like your personal identification number (PIN), your password is your secret; if you choose your password well, it's likely difficult for others to guess it at random. The combination of your name and a strong password provide sufficient corroboration.
Of course, strong passwords can be difficult to remember, and the strain only worsens as you collect and memorize yet another eight-character key (of, say, numbers, punctuation, and mixed case). Typing a password over and over again can be downright annoying -- worse, it presents a significant obstacle to hands-off automation.
Recognizing this encumbrance, many command-line utilities allow you to provide your username and password as command-line arguments. For example, you can log in to an FTP site without intervention by using a command such as:
ftp ftp://joe:passwd@www.example.com
However, using such a facility can reveal your credentials to other users
sharing your computer. (Try ps -Aeww, for example, to see the
complete command line and environment of every process on the system.)
To provide the same convenience as command-line options without the
inherent risks, many programs can read your credentials from a special
file called a .netrc (pronounced net-r-c) file, which
typically resides in ~/.netrc. Your .netrc file must be owner read-write
mode (mode 0600 or -rw-------) only, and each
entry in the file must adhere to this simple syntax:
machine ftp.example.com login zaphod password I()Trillian! machine www.magazine.com login abner password MmG8y*tr default login anonymous password zaphod@heartofgold.com
The first two lines provide the machine keyword and the
computer's domain name, the login keyword and your login name
on the computer, and the password keyword followed by the
password associated with your login. The credentials on the last line
provide a default for any system not specifically named. The
default line must be the last line in your .netrc file. (For
the full extent of .netrc file configuration options, type
man 5 netrc to see the .netrc man page.)
Obviously, if any file has any identity data in it, secure it with user
read-write only mode (mode 600) or user read-only mode (mode
400) to prevent you from accidentally overwriting or removing
it. You might also want to protect your home directory with mode
700.
Now, whenever you launch a .netrc-enabled application, including those
applications I discuss next, the appropriate login name and password are
passed automatically to the desired service, with nary a single peck at
the keyboard. You can typically disable this auto-login feature
with the -n option.
The process of transference
Along with HTTP and HTTP over SSL (HTTPS) for Web pages, FTP is one of the most often used Internet application protocols. Through FTP, a client can connect to a server, acquire a list of directories and files, and either download a file (that is, request a file from the server) or upload a file (that is, send the file a server to persist). URLs of the forms ftp://ftp.example.com/path/to/anotherfile.zip and ftp://user:password@ftp.example.com/path/to/file.zip imply, using the FTP protocol, connect to ftp.example.com and download the file /path/to/anotherfile.zip. The latter URL simply adds credentials for login.
On most desktop computers, such URLs launch the browser or the default FTP
application to download the specified file. However, you can use the same
URLs with the wget command-line utility -- a robust utility
for downloading files over HTTP, HTTPS, and FTP. It supports .netrc files
and is entirely non-interactive, making it ideal for automation. If your
system does not have wget, you can download its source code
from the GNU Software
Foundation. It builds readily on all UNIX variants with just a few
simple commands, and you can place the utility in your personal
bin directory or in a central directory.
Assuming that you have a .netrc file in place, let's look at some examples
of what wget can do. (In the examples below, the line number
is provided for reference; you don't need to type the numbers.) Listing 1 shows how to use wget to
download files without leaving the comfort of the command line.
Listing 1. Using wget to download files at the command line
1 $ wget http://ftp.gnu.org/pub/gnu/wget/wget-1.10.2.tar.gz
--16:02:29-- http://ftp.gnu.org/pub/gnu/wget/wget-1.10.2.tar.gz
=> `wget-1.10.2.tar.gz'
Resolving ftp.gnu.org... 199.232.41.7
Connecting to ftp.gnu.org[199.232.41.7]:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 1,213,056 [application/x-tar]
100%[=====================>] 1,213,056 531.22K/s
16:02:37 (529.57 KB/s) - `wget-1.10.2.tar.gz' saved [1213056/1213056]
2 $ wget -q ftp://mirror.linux.duke.edu/pub/centos/4.3/os\
/i386/RELEASE-NOTES-en.html
3 $ cat url_list.txt
http://www.wikipedia.com
http://valdez.barebones.com/pub/freeware/TextWrangler_2.1.3.dmg
4 $ wget -i -nv url_list.txt
16:06:00 URL:http://www.wikipedia.org/ [33606] -> "index.html" [1]
16:06:41 URL:http://valdez.barebones.com/pub/freeware/
TextWrangler_2.1.3.dmg [9488296/9488296] ->
"TextWrangler_2.1.3.dmg" [1]
FINISHED --16:06:41--
Downloaded: 9,521,902 bytes in 2 files
5 $ ls
RELEASE-NOTES-en.html index.html wget-1.10.2.tar.gz
TextWrangler_2.1.3.dmg url_list.txtCommand 1 downloads the most recent wget source code from its
project home page over HTTP. By default, wget apprises you of
progress. You can disable all messages with the -q (for
quiet mode) option. Command 2 retrieves a version of the
CentOS release notes over FTP, albeit very quietly.
If you have a long list of URLs to download, you need not place each one on
the command line. Instead, you can create (or rather, generate) a list of
URLs to download. Command 3 shows url_list.txt, a simple text catalog
containing two URLs; Command 4 downloads the two URLs. Use the
-i option when you provide a list. The -nv
option -- an acronym for not verbose -- provides more concise
messages.
Unless you provide a file name for the download file (using the
-o option), wget creates a new, local file with
the same name as the remote file, omitting the entire leading URL. Command
5 shows the four files downloaded in commands 1 through 3.
The wget utility has many options and features. It can
spider an FTP or Web site and download a complete hierarchy
of files. You can also set a quota for automatic downloads, provide
cookies, and continue a previous download that was interrupted. Read the
wget man page to learn about the tool's many tricks.
Going up
The wget utility is invaluable for hands-off downloads, but it
can't upload files. Nor can it interoperate with secure FTP,
telnet, and a host of other (older and less-used) Internet
protocols. For those kinds of transfers, you must turn to the veritable
Swiss Army knife of networking: curl.
The curl command-line utility can get and put data, so it's
ideal for transferring local files to remote servers. Better yet, the
underpinning of curl -- the libcurl library -- has a rich
application programming interface (API) that allows you to interrogate all
the features of curl directly into your own applications. The
C, C++, PHP, and Perl programming languages are
just four of the many languages that can leverage libcurl. If your system
lacks curl and libcurl, you can download the source code from
the libcurl home page.
Because curl can copy local files to remote servers, it's
ideal for small backups. For example, Listing 2 shows
a shell script that copies a directory full of database dumps to a remote
FTP server for safekeeping.
Listing 2. Using curl to store database dumps remotely
foreach db (mydns mysql cms tv radio) /usr/bin/mysqldump --ppassword --add-drop-table -Q --complete-insert $db > $db.sql end find dbs -mtime -1 -type f -name '*.sql' -print | foreach file (`xargs`) curl -n -T $file ftp://ftp1.archive.example.com end
The curl -n command forces curl to read your
.netrc file. The -T option tells curl to upload
the named file(s) to the given URL. If you omit the target file name,
curl simply reuses the name of the file being uploaded.
As you might guess, curl has even more options than
wget. It's worthwhile to read the curl man page
and keep it in mind. The curl project also maintains a list of uses,
including instructions on how to use the HTTP POST and
PUT commands, how to provide login credentials, how to use
SSL certificates, and how to debug your curl requests. A
quick tip: Try curl -v --trace-ascii ... to generate tracing
information.
Six addresses of separation
Modern computing depends largely on countless, spindly interconnections among machines of all shapes, sizes, and services. Indeed, even in a small computing environment, one computer might be dedicated to e-mail, another to serving Web pages, and others to performing yet more specialized tasks. In this environment -- typically connected by a local area network (LAN), WAN, or Virtual Private Network (VPN) -- it's quite common and necessary to log in to several computers per day. Systems administrators bounce from one computer to another each and every hour, but it's common for developers and other users to log in to require remote access for a critical application.
The X Window System and current desktop software make remote access fairly
transparent: A window is a window, and the underlying application could be
running on any computer. But again, the command line has a special place,
even in a mouse-centric world. For example, how can you run the same
command on multiple computers painlessly? Or, more simply, how do you
launch an xterm window on the remote system?
Remote system access is the purview of ssh and its
derivatives, scp and sftp. ssh is
the secure version of rsh, while scp and
sftp are secure replacements for rcp and FTP,
respectively. Why is it secure? ssh and its variants provide
stronger authentication mechanisms and encrypt all traffic using your
choice of several ciphers. Even if someone sniffed your network,
ssh traffic would look like so much gobblygook.
The simplest use of ssh is ssh hostname.
This command connects to hostname and presents you with a login
and password prompt. Provide the right credentials, and you're in:
(www.joe.com) $ ssh web.example.com Login: arthur Password: ****** ( web.example.com) $
If you simply want to run one command on a remote system, you don't need to
log in. Just provide the command as an argument to ssh. For
example, the command shown in Listing 3 runs
hostname -a -v on the remote computer.
Listing 3. Use ssh to run a command on a remote system
(www.joe.com) $ ssh db.linux-mag.com hostname -a -v Login: vogon Password: ****** db gethostname()=`db.linux-mag.com' Resolving `db.linux-mag.com' ... Result: h_name=`db.linux-mag.com' Result: h_aliases=`db' Result: h_addr_list=`64.34.170.230'
ssh opened a connection to db.linux-mag.com and passed the
hostname -a -v arguments to the remote computer, which ran
the command and returned the output to the local computer.
ssh also provides a convenient way to copy files and entire
directories from one computer to another. scp is almost as
easy to use as cp. Here's an example:
(www.joe.com) $ scp -p -r ~/myproject web.example.com:
This command copies the ~/myproject directory to web.example.com. If you
omit a destination path name, files are copied to your home directory. The
-p option preserves the date and time stamps on all the
files, while -r enables recursive mode, where
scp descends and copies all subdirectories, as well.
By the way, the previous scp command is the equivalent of
running:
(www.joe.com) $ tar czf - ~/myproject | ssh www.example.com tar xvzf - Login: deepthought Password: ******
Yes, you can pipe the output of a local command to a remote command (or vice versa).
Chances are, you're already tired of all those password prompts. Again, the
repeated prompts simply slow down work and prevent automation. You might
also be tired of typing long host names over and over again. Luckily,
ssh supports public or private key authentication and system
aliases.
Let's set up a public or private key pair using the DSA encryption scheme. To do so, you must generate the key pair, copy the public key to the remote system, add it to the list of known keys, and verify that everything works, as shown in Listing 4.
Listing 4. Creating and installing a public or private key
1 $ cd ~ 2 $ mkdir .ssh 3 $ chmod 700 .ssh 4 $ cd .ssh 5 $ ssh-keygen -t dsa Generating public/private dsa key pair. Enter file in which to save the key (/home/mstreicher/.ssh/id_dsa): ./id_dsa Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in ./id_dsa. Your public key has been saved in ./id_dsa.pub. The key fingerprint is: 40:6c:26:e7:53:df:d1:7b:c4:79:c5:a8:cd:6b:fe:8e mstreicher@db.linux-mag.com 6 $ ls id_dsa id_dsa.pub 7 $ chmod 600 * 8 $ scp id_dsa.pub www.example.com: Login: marvin Password: ****** id_dsa 100% 668 0.7KB/s 00:00 9 $ ssh www.example.com Login: marvin Password: ****** A $ mkdir .ssh B $ chmod 700 .ssh C $ cd .ssh D $ cat ../id_dsa.pub >> authorized_keys E $ rm ../id_dsa.pub F $ chmod 600 * G $ logout 10 $ ssh www.example.com a $ hostname www.example.com b $ logout
Commands 1 through 3 create a private local directory named .ssh in your
home directory. This directory must be mode 700, or
ssh won't use public or private key authentication. (You can
see the same sequence of commands run on the remote computer in steps A
through C.) Command 5 creates the key pair using DSA. For now, leave the
two passphrases blank. (They provide an extra level of security but add
another authentication step.) ssh-keygen generates two files:
id_dsa (the private key) and id_dsa.pub (the public key). Step 6 shows the
files, while Step 7 protects both keys. Your keys must be mode
0600 or mode 0400.
Step 8 copies the public key to the remote computer. For now, you must type your password, but this is the last time. Commands A through C create the private .ssh directory, and Step D adds the public key to the list of authorized keys. The name of the file -- authorized_keys -- is intentional. Do not name the file differently. Step E removes the copy of the key; Step F protects the files, as in Step 7.
When you log out and log back in, a password is no longer required.
ssh (and scp and sftp) tests your
private key against the remote public key. If a match is found,
your credentials are sound and you can log in without further
identification.
Some systems will always require a password; other systems might prefer RSA over DSA. Check with your systems administrator to find out how to log in to a specific computer. Systems administrators can set some global settings, too, to make a system more accessible.
Online, everywhere, all the time
Nowadays, the Internet connects people far and wide in ways not experienced before in human history. Whether sharing the details of your day in your blog or downloading source code for your next project, wires have replaced tires as the way to get around.
Web surfing is still a popular sport, but to make time for real surfing, developers have created ways to automate file transfers of all kinds. Using scripts and a few UNIX utilities, you can keep your external Web and download sites current. You can download and upload files with just a few keystrokes, making the process quick and easy. And if you create a .netrc file, you can hasten the effort even more. No more passwords.
Now that your mind is clear, put the top down and take a road trip on the information superhighway. See you at the Restaurant at the End of the Fiber. Last one there picks up the tab!
Downloadable resources
Related topics
- Speaking UNIX: Check out other parts in this series.
curl: Discover clever ways to use Curl to download a variety of resources, using many of the most common Internet protocols.- wget: Download the source code for the wget utility from the GNU Software Foundation.
curlutility and the libcurl library: Download the source code for the curl utility and libcurl library.- IBM trial software: Build your next development project with software for download directly from developerWorks.