external.html

Shall高级编程

></P
><P
>The <I
CLASS="FIRSTTERM"
>rm</I
> command will, by
		itself,  fail to remove filenames beginning with
		a dash. Why? Because <I
CLASS="FIRSTTERM"
>rm</I
>
		sees a dash-prefixed filename as an
		<I
CLASS="FIRSTTERM"
>option</I
>.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rm -badname</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>rm: invalid option -- b
 Try `rm --help' for more information.</TT
></PRE
></TD
></TR
></TABLE
>
	      </P
><P
>		One clever workaround is to precede
		the filename with a <SPAN
CLASS="QUOTE"
>" -- "</SPAN
> (the
		<I
CLASS="FIRSTTERM"
>end-of-options</I
> flag).

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rm -- -badname</B
></TT
></PRE
></TD
></TR
></TABLE
>
		</P
><P
>	      Another method to is to preface the filename to be removed
	      with a <TT
CLASS="FILENAME"
>dot-slash</TT
> .

	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rm ./-badname</B
></TT
></PRE
></TD
></TR
></TABLE
>
	      </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="WARNING"
><TABLE
CLASS="WARNING"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/warning.png"
HSPACE="5"
ALT="Warning"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="RMRECURS"
></A
>When used with the
	      recursive flag <TT
CLASS="OPTION"
>-r</TT
>, this command removes
	      files all the way down the directory tree from the current
	      directory.  A careless <B
CLASS="COMMAND"
>rm -rf *</B
> can wipe
	      out a big chunk of a directory structure.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="RMDIRREF"
></A
><B
CLASS="COMMAND"
>rmdir</B
></DT
><DD
><P
>Remove directory. The directory must be empty of
	      all files -- including <SPAN
CLASS="QUOTE"
>"invisible"</SPAN
>
	      <I
CLASS="FIRSTTERM"
>dotfiles</I
>

		<A
NAME="AEN9301"
HREF="#FTN.AEN9301"
>[1]</A
>

	      -- for this command to succeed.</P
></DD
><DT
><A
NAME="MKDIRREF"
></A
><B
CLASS="COMMAND"
>mkdir</B
></DT
><DD
><P
>Make directory, creates a new directory. For example,
	      <TT
CLASS="USERINPUT"
><B
>mkdir -p project/programs/December</B
></TT
>
	      creates the named directory. The
	      <TT
CLASS="REPLACEABLE"
><I
>-p</I
></TT
> option automatically creates
	      any necessary parent directories.</P
></DD
><DT
><A
NAME="CHMODREF"
></A
><B
CLASS="COMMAND"
>chmod</B
></DT
><DD
><P
>Changes the attributes of an existing file or directory
	      (see <A
HREF="internal.html#EX44"
>Example 14-13</A
>).</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;chmod +x filename
   2&nbsp;# Makes "filename" executable for all users.
   3&nbsp;
   4&nbsp;chmod u+s filename
   5&nbsp;# Sets "suid" bit on "filename" permissions.
   6&nbsp;# An ordinary user may execute "filename" with same privileges as the file's owner.
   7&nbsp;# (This does not apply to shell scripts.)</PRE
></TD
></TR
></TABLE
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;chmod 644 filename
   2&nbsp;# Makes "filename" readable/writable to owner, readable to others
   3&nbsp;# (octal mode).
   4&nbsp;
   5&nbsp;chmod 444 filename
   6&nbsp;#  Makes "filename" read-only for all.
   7&nbsp;#  Modifying the file (for example, with a text editor)
   8&nbsp;#+ not allowed for a user who does not own the file (except for root),
   9&nbsp;#+ and even the file owner must force a file-save
  10&nbsp;#+ if she modifies the file.
  11&nbsp;#  Same restrictions apply for deleting the file.</PRE
></TD
></TR
></TABLE
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;chmod 1777 directory-name
   2&nbsp;#  Gives everyone read, write, and execute permission in directory,
   3&nbsp;#+ however also sets the "sticky bit".
   4&nbsp;#  This means that only the owner of the directory,
   5&nbsp;#+ owner of the file, and, of course, root
   6&nbsp;#+  can delete any particular file in that directory.
   7&nbsp;
   8&nbsp;chmod 111 directory-name
   9&nbsp;#  Gives everyone execute-only permission in a directory.
  10&nbsp;#  This means that you can execute and READ the files in that directory
  11&nbsp;#+ (execute permission necessarily includes read permission
  12&nbsp;#+ because you can't execute a file without being able to read it).
  13&nbsp;#  But you can't list the files or search for them with the "find" command.
  14&nbsp;#  These restrictions do not apply to root.
  15&nbsp;
  16&nbsp;chmod 000 directory-name
  17&nbsp;#  No permissions at all for that directory.
  18&nbsp;#  Can't read, write, or execute files in it.
  19&nbsp;#  Can't even list files in it or "cd" to it.
  20&nbsp;#  But, you can rename (mv) the directory
  21&nbsp;#+ or delete it (rmdir) if it is empty.
  22&nbsp;#  You can even symlink to files in the directory,
  23&nbsp;#+ but you can't read, write, or execute the symlinks.
  24&nbsp;#  These restrictions do not apply to root.</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="CHATTRREF"
></A
><B
CLASS="COMMAND"
>chattr</B
></DT
><DD
><P
><B
CLASS="COMMAND"
>Ch</B
>ange file
	      <B
CLASS="COMMAND"
>attr</B
>ibutes. This is analogous to
	      <B
CLASS="COMMAND"
>chmod</B
> above, but with different options
	      and a different invocation syntax, and it works only on
	      an <I
CLASS="FIRSTTERM"
>ext2</I
> filesystem.</P
><P
>One particularly interesting <B
CLASS="COMMAND"
>chattr</B
>
	      option is <TT
CLASS="OPTION"
>i</TT
>. A <B
CLASS="COMMAND"
>chattr +i
	      <TT
CLASS="FILENAME"
>filename</TT
></B
> marks the file
	      as immutable. The file cannot be modified, linked to, or
	      deleted, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not even by root</I
></SPAN
>. This
	      file attribute can be set or removed only by
	      <I
CLASS="FIRSTTERM"
>root</I
>. In a similar fashion,
	      the <TT
CLASS="OPTION"
>a</TT
> option marks the file as append
	      only.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>chattr +i file1.txt</B
></TT
>
 
 
 <TT
CLASS="PROMPT"
>root# </TT
><TT
CLASS="USERINPUT"
><B
>rm file1.txt</B
></TT
>
 
 <TT
CLASS="COMPUTEROUTPUT"
>rm: remove write-protected regular file `file1.txt'? y
 rm: cannot remove `file1.txt': Operation not permitted</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
><P
>If a file has the <TT
CLASS="OPTION"
>s</TT
> (secure)
              attribute set, then when it is deleted its block is zeroed out
	      on the disk.</P
><P
>If a file has the <TT
CLASS="OPTION"
>u</TT
> (undelete)
              attribute set, then when it is deleted, its contents can still
	      be retrieved (undeleted).</P
><P
>If a file has the <TT
CLASS="OPTION"
>c</TT
> (compress)
              attribute set, then it will automatically be compressed
	      on writes to disk, and uncompressed on reads.</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
>The file attributes set with
	      <B
CLASS="COMMAND"
>chattr</B
> do not show in a file listing
	      (<B
CLASS="COMMAND"
>ls -l</B
>).</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="LINKREF"
></A
><B
CLASS="COMMAND"
>ln</B
></DT
><DD
><P
>Creates links to pre-existings files. A <SPAN
CLASS="QUOTE"
>"link"</SPAN
>
	      is a reference to a file, an alternate name for it.
	      The <B
CLASS="COMMAND"
>ln</B
> command permits referencing
	      the linked file by more than one name and is a superior
	      alternative to aliasing (see <A
HREF="othertypesv.html#EX18"
>Example 4-6</A
>).</P
><P
>The <B
CLASS="COMMAND"
>ln</B
> creates only a reference, a
	       pointer to the file only a few bytes in size.</P
><P
><A
NAME="SYMLINKREF"
></A
></P
><P
>The <B
CLASS="COMMAND"
>ln</B
> command is most often used
	      with the <TT
CLASS="OPTION"
>-s</TT
>, symbolic or
	      <SPAN
CLASS="QUOTE"
>"soft"</SPAN
> link flag. Advantages of using the
	      <TT
CLASS="OPTION"
>-s</TT
> flag are that it permits linking across
	      file systems or to directories.</P
><P
>The syntax of the command is a bit tricky. For example:
	      <TT
CLASS="USERINPUT"
><B
>ln -s oldfile newfile</B
></TT
> links the
	      previously existing <TT
CLASS="FILENAME"
>oldfile</TT
> to the
	      newly created link, <TT
CLASS="FILENAME"
>newfile</TT
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>If a file named <TT
CLASS="FILENAME"
>newfile</TT
> has
	      previously existed, an error message will
	      result.</P
></TD
></TR
></TABLE
></DIV
><TABLE
CLASS="SIDEBAR"
BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN9407"
></A
><P
><B
>Which type of link to use?</B
></P
><P
>As John Macdonald explains it:</P
><P
>Both of these [types of links] provide a certain measure of dual reference
		-- if you edit the contents of the file using any name,
		your changes will affect both the original name and either
		a hard or soft new name.  The differences between them
		occurs when you work at a higher level.  The advantage of
		a hard link is that the new name is totally independent
		of the old name -- if you remove or rename the old name,
		that does not affect the hard link, which continues
		to point to the data while it would leave a soft link
		hanging pointing to the old name which is no longer
		there. The advantage of a soft link is that it can refer
		to a different file system (since it is just a reference
		to a file name, not to actual data). And, unlike a hard
		link, a symbolic link can refer to a directory.</P
></DIV
></TD
></TR
></TABLE
><P
><A
NAME="LINKMINVOK"
></A
></P
><P
>Links give the ability to invoke a script (or any other type
	      of executable) with multiple names, and having that script
	      behave according to how it was invoked.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="HELLOL"
></A
><P
><B
>Example 15-2. Hello or Good-bye</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# hello.sh: Saying "hello" or "goodbye"
   3&nbsp;#+          depending on how script is invoked.
   4&nbsp;
   5&nbsp;# Make a link in current working directory ($PWD) to this script:
   6&nbsp;#    ln -s hello.sh goodbye
   7&nbsp;# Now, try invoking this script both ways:
   8&nbsp;# ./hello.sh
   9&nbsp;# ./goodbye
  10&nbsp;
  11&nbsp;
  12&nbsp;HELLO_CALL=65
  13&nbsp;GOODBYE_CALL=66
  14&nbsp;
  15&nbsp;if [ $0 = "./goodbye" ]
  16&nbsp;then
  17&nbsp;  echo "Good-bye!"
  18&nbsp;  # Some other goodbye-type commands, as appropriate.
  19&nbsp;  exit $GOODBYE_CALL
  20&nbsp;fi
  21&nbsp;
  22&nbsp;echo "Hello!"
  23&nbsp;# Some other hello-type commands, as appropriate.
  24&nbsp;exit $HELLO_CALL</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="MANREF"
></A
><B
CLASS="COMMAND"
>man</B
>, <A
NAME="INFOREF"
></A
><B
CLASS="COMMAND"
>info</B
></DT
><DD
><P
>These commands access the manual and information pages on
	      system commands and installed utilities. When available, the
	      <I
CLASS="FIRSTTERM"
>info</I
> pages usually contain more detailed
	      descriptions than do the <I
CLASS="FIRSTTERM"
>man</I
> pages.</P
></DD
></DL
></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN9301"
HREF="external.html#AEN9301"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="DOTFILESREF"
></A
></P
><P
><I
CLASS="FIRSTTERM"
>Dotfiles</I
> are files whose
		    names begin with a <I
CLASS="FIRSTTERM"
>dot</I
>, such as
		    <TT
CLASS="FILENAME"
>~/.Xdefaults</TT
>. Such filenames do
		    not appear in a normal <B
CLASS="COMMAND"
>ls</B
> listing
		    (although an <B
CLASS="COMMAND"
>ls -a</B
> will show
		    them), and they cannot be deleted by an accidental
		    <B
CLASS="COMMAND"
>rm -rf *</B
>. Dotfiles are generally
		    used as setup and configuration files in a user's
		    home directory.</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="internal.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="moreadv.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Internal Commands and Builtins</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="part4.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Complex Commands</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>