shell中使用ftp

有时开发或测试的时候需要反复的从ftp上下载某个文件，每次都需要输入用户名及密码。如果能够避免ftp下载中的交互，用一个脚本实现，那会方便多了。正好网上找到一篇文章，是讲如何在shell中使用ftp下载。转载过来，以飨大家。

How to use ftp in a shell script

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Sometimes I want to FTP a file from one machine to another. Usually, I can do the transfer interactively, but every so often, I would like to have a shell script do the file transfer. This task has eluded me in the past, but I finally figured it out. I've not seen this particular trick documented in the past, so I submit it for your approval.

The Problem

The problem I always encountered in scripting ftp transfers involved getting a password to the ftp server. Typical ftp client programs under Unix, Linux, Solaris and NetBSD all read the ftp password from /dev/tty.

Example (non-working) script

#!/bin/sh
HOST='ftp.users.qwest.net'
USER='yourid'
PASSWD='yourpw'
FILE='file.txt'

ftp $HOST <user $USER
$PASSWD
put $FILE
quit
END_SCRIPT
exit 0

The above script will just hang if run in the foreground (in an xterm), or if run in the background (from a cron job), it will fail to perform the work of transferring file.txt.

/dev/tty names a strange, magic device. Each process (more strictly each process group) has a different /dev/tty, and you can not naively make ftp clients read the password from some non-magic, yet convenient source, like a "here document". When run in an xterm, the script above appears to hang because it reads the password from /dev/tty. The xterm constitutes the script's /dev/tty, so the script waits for keyboard input.

Example Working Script

#!/bin/sh
HOST='ftp.users.qwest.net'
USER='yourid'
PASSWD='yourpw'
FILE='file.txt'

ftp -n $HOST <quote USER $USER
quote PASS $PASSWD
put $FILE
quit
END_SCRIPT
exit 0

The Tricks

Getting the password to the ftp server without having the ftp client program read the password from /dev/tty requires two tricks:

1. Using the -n option on the ftp client program to prevent the ftp client from trying to log in immediately. That way, the ftp client does not ask for a user ID and password. No use of /dev/tty.
2. Use the ftp client program command quote to send user ID and password to the ftp server.

Further Refinements

The above sh script will spew lots of ftp client output to standard output. Even if everything works perfectly, the user running the above script will see lots of incomprehensible text scrolling by quite rapidly. One refinement would send output to different places:

ftp -n $HOST > /tmp/ftp.worked 2> /tmp/ftp.failed <

One could further refine error handling by acting on the ftp client program's exit status:

ftp -n $HOST > /tmp/ftp.worked 2> /tmp/ftp.failed <blah blah
END_SCRIPT

EXITSTATUS=$?

if [ $EXITSTATUS != "0" ]
then
    # handle the error...
fi 

Except that the above doesn't always work - most FTP clients always exit with a status of 0. This leads to ugly "false negatives": the file transfer fails, but the script doesn't detect the problem.

One way to verify that a file transfer took place - transfer it back:

#!/bin/sh

ftp -n << END_SCRIPT
open $1
user $2 $3
put $4
get $4 retrieval.$$
bye
END_SCRIPT

if [ -f retrieval.$$ ]
then
	echo "FTP of $4 to $1 worked"
	rm -f retrieval.$$
else
	echo "FTP of $4 did not work"
fi

Regular FTPs there and back of large files can consume a lot of time.

Control of ftp by a shell script

One obvious improvement would have the ftp client program controlled by the shell script. I don't think that would comprise an impossible task, but I also don't think that it would have much value. Scripting ftp transfer using might cause you less pain.

---

Alternative #1

I saw a second way of doing this in a usenet article:

#!/bin/sh
USER=userid
PASSWD=userpw
ftp -n f2dev <user $USER $PASSWD
binary
get some.file
quit
SCRIPT

It still uses the "-n" trick, but it sends user ID and password in the same "user" command.

---

Alternative #2

Use a .netrc file

Linux, Unix and BSD users have the alternative of using a .netrc file. The ftp man page documents the format of .netrc. To accomplish the task of using ftp in a shell script you would have to fill out a .netrc file something like this:

machine something.else.com
login myid
password mypassword

ftp demands that .netrc not have group or world read or write permissions:

$ ls -l .netrc
-rw-------    1 bediger  users          51 Dec 16 13:30 .netrc

Using a .netrc file has a few problems that may or may not prevent you from using it.

• A shell scripkt that does FTP using .netrc is no longer self-contained. You have to keep track of two files, which means that bugs can be less than obvious.
• ftp reads it's user ID's .netrc. If you develop your script under a given user ID, then put it in production under a second user ID, you have to coordinate .netrc file contents between those two user IDs.

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Alternative #3

Apparently, the program from Columbia University understands FTP. You could use Ckermit to . This looks to have advantages and disadvantages. On the "pro" side, it appears that Ckermit can exit on various errors, like unknown user IDs, or bad passwords. On the "con" side, you have to have Ckermit. I don't recall that it had a too onerous install, but it doesn't come with many Linux distros these days, and it probably doesn't come with any vendor Unix.