system.html

Shall高级编程

>ubstitute <B
CLASS="COMMAND"
>u</B
>ser.
	      <B
CLASS="COMMAND"
>su rjones</B
> starts a shell as user
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>rjones</I
></SPAN
>. A naked <B
CLASS="COMMAND"
>su</B
>
	      defaults to <I
CLASS="FIRSTTERM"
>root</I
>.  See <A
HREF="contributed-scripts.html#FIFO"
>Example A-15</A
>.</P
></DD
><DT
><A
NAME="SUDOREF"
></A
><B
CLASS="COMMAND"
>sudo</B
></DT
><DD
><P
>Runs a command as <I
CLASS="FIRSTTERM"
>root</I
> (or
	      another user). This may be used in a script, thus permitting
	      a <I
CLASS="FIRSTTERM"
>regular user</I
> to run the script.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;# Some commands.
   4&nbsp;sudo cp /root/secretfile /home/bozo/secret
   5&nbsp;# Some more commands.</PRE
></TD
></TR
></TABLE
></P
><P
>The file <TT
CLASS="FILENAME"
>/etc/sudoers</TT
> holds
	      the names of users permitted to invoke
	      <B
CLASS="COMMAND"
>sudo</B
>.</P
></DD
><DT
><A
NAME="PASSWDREF"
></A
><B
CLASS="COMMAND"
>passwd</B
></DT
><DD
><P
>Sets, changes, or manages a user's password.</P
><P
>The <B
CLASS="COMMAND"
>passwd</B
> command can be used in
	      a script, but probably <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>should not</I
></SPAN
> be.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SETNEWPW"
></A
><P
><B
>Example 16-1. Setting a new password</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;#  setnew-password.sh: For demonstration purposes only.
   3&nbsp;#                      Not a good idea to actually run this script.
   4&nbsp;#  This script must be run as root.
   5&nbsp;
   6&nbsp;ROOT_UID=0         # Root has $UID 0.
   7&nbsp;E_WRONG_USER=65    # Not root?
   8&nbsp;
   9&nbsp;E_NOSUCHUSER=70
  10&nbsp;SUCCESS=0
  11&nbsp;
  12&nbsp;
  13&nbsp;if [ "$UID" -ne "$ROOT_UID" ]
  14&nbsp;then
  15&nbsp;  echo; echo "Only root can run this script."; echo
  16&nbsp;  exit $E_WRONG_USER
  17&nbsp;else
  18&nbsp;  echo
  19&nbsp;  echo "You should know better than to run this script, root."
  20&nbsp;  echo "Even root users get the blues... "
  21&nbsp;  echo
  22&nbsp;fi  
  23&nbsp;
  24&nbsp;
  25&nbsp;username=bozo
  26&nbsp;NEWPASSWORD=security_violation
  27&nbsp;
  28&nbsp;# Check if bozo lives here.
  29&nbsp;grep -q "$username" /etc/passwd
  30&nbsp;if [ $? -ne $SUCCESS ]
  31&nbsp;then
  32&nbsp;  echo "User $username does not exist."
  33&nbsp;  echo "No password changed."
  34&nbsp;  exit $E_NOSUCHUSER
  35&nbsp;fi  
  36&nbsp;
  37&nbsp;echo "$NEWPASSWORD" | passwd --stdin "$username"
  38&nbsp;#  The '--stdin' option to 'passwd' permits
  39&nbsp;#+ getting a new password from stdin (or a pipe).
  40&nbsp;
  41&nbsp;echo; echo "User $username's password changed!"
  42&nbsp;
  43&nbsp;# Using the 'passwd' command in a script is dangerous.
  44&nbsp;
  45&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>The <B
CLASS="COMMAND"
>passwd</B
> command's <TT
CLASS="OPTION"
>-l</TT
>,
	      <TT
CLASS="OPTION"
>-u</TT
>, and <TT
CLASS="OPTION"
>-d</TT
> options permit
	      locking, unlocking, and deleting a user's password. Only
	      <I
CLASS="FIRSTTERM"
>root</I
> may use these options.</P
></DD
><DT
><A
NAME="ACREF"
></A
><B
CLASS="COMMAND"
>ac</B
></DT
><DD
><P
>Show users' logged in time, as read from
	      <TT
CLASS="FILENAME"
>/var/log/wtmp</TT
>. This is one of the GNU
	      accounting utilities.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ac</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>        total       68.08</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="LASTREF"
></A
><B
CLASS="COMMAND"
>last</B
></DT
><DD
><P
>List <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>last</I
></SPAN
> logged in users, as read from
	      <TT
CLASS="FILENAME"
>/var/log/wtmp</TT
>. This command can also
	      show remote logins.</P
><P
>For example, to show the last few times the system
	      rebooted:</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>last reboot</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>reboot   system boot  2.6.9-1.667      Fri Feb  4 18:18          (00:02)    
 reboot   system boot  2.6.9-1.667      Fri Feb  4 15:20          (01:27)    
 reboot   system boot  2.6.9-1.667      Fri Feb  4 12:56          (00:49)    
 reboot   system boot  2.6.9-1.667      Thu Feb  3 21:08          (02:17)    
 . . .

 wtmp begins Tue Feb  1 12:50:09 2005</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="NEWGRPREF"
></A
><B
CLASS="COMMAND"
>newgrp</B
></DT
><DD
><P
>Change user's <I
CLASS="FIRSTTERM"
>group ID</I
> without
	      logging out. This permits access to the new group's
	      files. Since users may be members of multiple groups
	      simultaneously, this command finds only limited use.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Kurt Glaesemann points out that the
	      <I
CLASS="FIRSTTERM"
>newgrp</I
> command could prove helpful
	      in setting the default group permissions for files a user
	      writes. However, the <A
HREF="system.html#CHGRPREF"
>chgrp</A
>
	      command might be more convenient for this purpose.</P
></TD
></TR
></TABLE
></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="TERMINALSSYS1"
></A
>Terminals</B
></P
><DL
><DT
><A
NAME="TTYREF"
></A
><B
CLASS="COMMAND"
>tty</B
></DT
><DD
><P
>Echoes the name of the current user's terminal.
	      Note that each separate <I
CLASS="FIRSTTERM"
>xterm</I
>
	      window counts as a different terminal.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>tty</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>/dev/pts/1</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="STTYREF"
></A
><B
CLASS="COMMAND"
>stty</B
></DT
><DD
><P
>Shows and/or changes terminal settings. This complex
	      command, used in a script, can control terminal behavior
	      and the way output displays. See the info page, and study
	      it carefully.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ERASE"
></A
><P
><B
>Example 16-2. Setting an <I
CLASS="FIRSTTERM"
>erase</I
> character</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# erase.sh: Using "stty" to set an erase character when reading input.
   3&nbsp;
   4&nbsp;echo -n "What is your name? "
   5&nbsp;read name                      #  Try to backspace
   6&nbsp;                               #+ to erase characters of input.
   7&nbsp;                               #  Problems?
   8&nbsp;echo "Your name is $name."
   9&nbsp;
  10&nbsp;stty erase '#'                 #  Set "hashmark" (#) as erase character.
  11&nbsp;echo -n "What is your name? "
  12&nbsp;read name                      #  Use # to erase last character typed.
  13&nbsp;echo "Your name is $name."
  14&nbsp;
  15&nbsp;exit 0
  16&nbsp;
  17&nbsp;# Even after the script exits, the new key value remains set.
  18&nbsp;# Exercise: How would you reset the erase character to the default value?</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SECRETPW"
></A
><P
><B
>Example 16-3. <I
CLASS="FIRSTTERM"
>secret password</I
>:
	      Turning off terminal echoing </B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# secret-pw.sh: secret password
   3&nbsp;
   4&nbsp;echo
   5&nbsp;echo -n "Enter password "
   6&nbsp;read passwd
   7&nbsp;echo "password is $passwd"
   8&nbsp;echo -n "If someone had been looking over your shoulder, "
   9&nbsp;echo "your password would have been compromised."
  10&nbsp;
  11&nbsp;echo &#38;&#38; echo  # Two line-feeds in an "and list."
  12&nbsp;
  13&nbsp;
  14&nbsp;stty -echo    # Turns off screen echo.
  15&nbsp;
  16&nbsp;echo -n "Enter password again "
  17&nbsp;read passwd
  18&nbsp;echo
  19&nbsp;echo "password is $passwd"
  20&nbsp;echo
  21&nbsp;
  22&nbsp;stty echo     # Restores screen echo.
  23&nbsp;
  24&nbsp;exit 0
  25&nbsp;
  26&nbsp;# Do an 'info stty' for more on this useful-but-tricky command.</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>A creative use of <B
CLASS="COMMAND"
>stty</B
> is detecting a
	      user keypress (without hitting
	      <B
CLASS="KEYCAP"
>ENTER</B
>).</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="KEYPRESS"
></A
><P
><B
>Example 16-4. Keypress detection</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# keypress.sh: Detect a user keypress ("hot keys").
   3&nbsp;
   4&nbsp;echo
   5&nbsp;
   6&nbsp;old_tty_settings=$(stty -g)   # Save old settings (why?).
   7&nbsp;stty -icanon
   8&nbsp;Keypress=$(head -c1)          # or $(dd bs=1 count=1 2&#62; /dev/null)
   9&nbsp;                              # on non-GNU systems
  10&nbsp;
  11&nbsp;echo
  12&nbsp;echo "Key pressed was \""$Keypress"\"."
  13&nbsp;echo
  14&nbsp;
  15&nbsp;stty "$old_tty_settings"      # Restore old settings.
  16&nbsp;
  17&nbsp;# Thanks, Stephane Chazelas.
  18&nbsp;
  19&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Also see <A
HREF="variables2.html#TIMEOUT"
>Example 9-3</A
>.</P
><P
><A
NAME="TERMINALSREF"
></A
></P
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN13777"
></A
><P
><B
>terminals and modes</B
></P
><P
>Normally, a terminal works in the
	      <I
CLASS="FIRSTTERM"
>canonical</I
> mode.  When a user hits a
	      key, the resulting character does not immediately go to
	      the program actually running in this terminal.  A buffer
	      local to the terminal stores keystrokes. When the user
	      hits the <B
CLASS="KEYCAP"
>ENTER</B
> key, this sends all the
	      stored keystrokes to the program running.  There is even
	      a basic line editor inside the terminal.</P
><P
>	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>stty -a</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>speed 9600 baud; rows 36; columns 96; line = 0;
 intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = &#60;undef&#62;; eol2 = &#60;undef&#62;;
 start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O;
 ...
 isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt</TT
>
                 </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>Using canonical mode, it is possible to redefine the
              special keys for the local terminal line editor.

	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat &#62; filexxx</B
></TT
>
 <TT
CLASS="USERINPUT"
><B
>wha&#60;ctl-W&#62;I&#60;ctl-H&#62;foo bar&#60;ctl-U&#62;hello world&#60;ENTER&#62;</B
></TT
>
 <TT
CLASS="USERINPUT"
><B
>&#60;ctl-D&#62;</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat filexxx</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>hello world</TT
>		
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>wc -c &#60; filexxx</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>12</TT
>		
                 </PRE
></TD
></TR
></TABLE
>

              The process controlling the terminal receives only 12
              characters (11 alphabetic ones, plus a newline), although
              the user hit 26 keys.
            </P
><P
>In non-canonical (<SPAN
CLASS="QUOTE"
>"raw"</SPAN
>) mode, every
              key hit (including special editing keys such as
              <B
CLASS="KEYCAP"
>ctl-H</B
>) sends a character immediately to
              the controlling process.</P
><P
>The Bash prompt disables both <TT
CLASS="OPTION"
>icanon</TT
>
              and <TT
CLASS="OPTION"
>echo</TT
>, since it replaces the basic
              terminal line editor with its own more elaborate one. For
              example, when you hit <B
CLASS="KEYCAP"
>ctl-A</B
> at the Bash
              prompt, there's no <B
CLASS="KEYCAP"
>^A</B
> echoed by the
              terminal, but Bash gets a <B
CLASS="KEYCAP"
>\1</B
> character,
              interprets it, and moves the cursor to the begining of
              the line.</P
><P
><SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>St閜hane Chazelas</I
></SPAN
></P
></DIV
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="SETTERMREF"
></A
><B
CLASS="COMMAND"
>setterm</B
></DT
><DD
><P
>Set certain terminal attributes. This command writes
	      to its terminal's <TT
CLASS="FILENAME"
>stdout</TT
> a string that
	      changes the behavior of that terminal.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>setterm -cursor off</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash$</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>The <B
CLASS="COMMAND"
>setterm</B
> command can be used within a
	      script to change the appearance of text written to
	      <TT
CLASS="FILENAME"
>stdout</TT
>, although there are certainly
	      <A
HREF="colorizing.html#COLORIZINGREF"
>better tools</A
> available
	      for this purpose.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;setterm -bold on
   2&nbsp;echo bold hello
   3&nbsp;
   4&nbsp;setterm -bold off
   5&nbsp;echo normal hello</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="TSETREF"
></A
><B
CLASS="COMMAND"
>tset</B
></DT
><DD
><P
>Show or initialize terminal settings. 
	      This is a less capable version of
	      <B
CLASS="COMMAND"
>stty</B
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR