分析一下这个简单shell (“自动”telnet)

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代码:

[color=blue]-(guest@mac:tty1)-(tmp)-
[25750 0] %[/color] cat telnet.sh
#!/bin/bash

if (( $# != 3 )); then
    echo "usage: $0 host user passwd"
    exit 1
fi

host=$1
user=$2
passwd=$3

inputfile=in
outputfile=out
rm -rf $inputfile
rm -rf $outputfile
mknod $inputfile p
touch $outputfile

exec 7<> $outputfile
exec 8<> $inputfile

telnet $host <&8 >&7 &

sleep 1
echo $user >> $inputfile
sleep 1
echo $passwd >> $inputfile

tail -f $outputfile &

while true; do
    read str
    if [[ $str = "quit" || $str = "exit" ]]
    then
        echo exit >> $inputfile
        sleep 1
        exit
    else
        echo $str >> $inputfile
    fi
done
[color=blue]-(guest@mac:tty1)-(tmp)-
[25750 0] %[/color] ./telnet.sh
usage: ./telnet.sh host user passwd
[color=blue]-(guest@mac:tty1)-(tmp)-
[25750 1] %[/color] ./telnet.sh localhost foo foo
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.

Linux 2.6.11-gentoo-r9 (localhost) (9)


tux login: foo
Password:
Last login: Wed Jun 22 22:51:08 from localhost
foo@tux foo $ pwd
pwd
/home/foo
foo@tux foo $ ls /
ls /
bin   dev  gentoo  lib         mnt  proc  sbin  tmp  var
boot  etc  home    lost+found  opt  root  sys   usr
foo@tux foo $ bye
bye
-bash: bye: command not found
foo@tux foo $ quit
Connection closed by foreign host.
exit
logout
[color=blue]-(guest@mac:tty1)-(tmp)-
[25750 0] %[/color]

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代码:

       exec [-cl] [-a name] [command [arguments]]
              If  command is specified, it replaces the shell.  No new process
              is created.  The arguments become the arguments to command.   If
              the -l option is supplied, the shell places a dash at the begin-
              ning of the zeroth arg passed to command.  This is what login(1)
              does.  The -c option causes command to be executed with an empty
              environment.  If -a is supplied, the shell passes  name  as  the
              zeroth  argument  to the executed command.  If command cannot be
              executed for some reason, a non-interactive shell exits,  unless
              the  shell  option execfail is enabled, in which case it returns
              failure.  An interactive shell returns failure if the file  can-
              not  be executed.  [color=red]If command is not specified, any redirections
              take effect in the current shell[/color], and the return  status  is  0.
              If there is a redirection error, the return status is 1.