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>Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting</TH
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="EXTERNAL"
></A
>Chapter 16. External Filters, Programs and Commands</H1
><P
><A
NAME="EXTERNALREF"
></A
></P
><P
>Standard UNIX commands make shell scripts more versatile. The
	power of scripts comes from coupling system commands and shell
	directives with simple programming constructs.</P
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="BASIC"
></A
>16.1. Basic Commands</H1
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="BASICCOMMANDS1"
></A
>The first commands a novice learns</B
></P
><DL
><DT
><A
NAME="LSREF"
></A
><B
CLASS="COMMAND"
>ls</B
></DT
><DD
><P
>The basic file <SPAN
CLASS="QUOTE"
>"list"</SPAN
> command. It is all too easy
	      to underestimate the power of this humble command. For
	      example, using the <TT
CLASS="OPTION"
>-R</TT
>, recursive option,
	      <B
CLASS="COMMAND"
>ls</B
> provides a tree-like listing of
	      a directory structure. Other useful options are
	      <TT
CLASS="OPTION"
>-S</TT
>, sort listing by file size,
	      <TT
CLASS="OPTION"
>-t</TT
>, sort by file modification time,
	      <TT
CLASS="OPTION"
>-v</TT
>, sort by (numerical) version numbers
	      embedded in the filenames,
                <A
NAME="AEN10025"
HREF="#FTN.AEN10025"
>[1]</A
>
	      <TT
CLASS="OPTION"
>-b</TT
>, show escape characters, and
	      <TT
CLASS="OPTION"
>-i</TT
>, show file inodes (see <A
HREF="moreadv.html#IDELETE"
>Example 16-4</A
>).</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -l</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>-rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -lv</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
> total 0
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
 -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt</TT
></PRE
></TD
></TR
></TABLE
></P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>	      The <I
CLASS="FIRSTTERM"
>ls</I
> command returns a
	      non-zero <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> when
	      attempting to list a non-existent file.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls abc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>ls: abc: No such file or directory</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $?</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>2</TT
></PRE
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="EX40"
></A
><P
><B
>Example 16-1. Using <I
CLASS="FIRSTTERM"
>ls</I
> to create a table of contents
		for burning a <SPAN
CLASS="ABBREV"
>CDR</SPAN
> disk</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# ex40.sh (burn-cd.sh)
   3&nbsp;# Script to automate burning a CDR.
   4&nbsp;
   5&nbsp;
   6&nbsp;SPEED=10         # May use higher speed if your hardware supports it.
   7&nbsp;IMAGEFILE=cdimage.iso
   8&nbsp;CONTENTSFILE=contents
   9&nbsp;# DEVICE=/dev/cdrom     For older versions of cdrecord
  10&nbsp;DEVICE="1,0,0"
  11&nbsp;DEFAULTDIR=/opt  # This is the directory containing the data to be burned.
  12&nbsp;                 # Make sure it exists.
  13&nbsp;                 # Exercise: Add a test for this.
  14&nbsp;
  15&nbsp;# Uses Joerg Schilling's "cdrecord" package:
  16&nbsp;# http://www.fokus.fhg.de/usr/schilling/cdrecord.html
  17&nbsp;
  18&nbsp;#  If this script invoked as an ordinary user, may need to suid cdrecord
  19&nbsp;#+ chmod u+s /usr/bin/cdrecord, as root.
  20&nbsp;#  Of course, this creates a security hole, though a relatively minor one.
  21&nbsp;
  22&nbsp;if [ -z "$1" ]
  23&nbsp;then
  24&nbsp;  IMAGE_DIRECTORY=$DEFAULTDIR
  25&nbsp;  # Default directory, if not specified on command-line.
  26&nbsp;else
  27&nbsp;    IMAGE_DIRECTORY=$1
  28&nbsp;fi
  29&nbsp;
  30&nbsp;# Create a "table of contents" file.
  31&nbsp;ls -lRF $IMAGE_DIRECTORY &#62; $IMAGE_DIRECTORY/$CONTENTSFILE
  32&nbsp;# The "l" option gives a "long" file listing.
  33&nbsp;# The "R" option makes the listing recursive.
  34&nbsp;# The "F" option marks the file types (directories get a trailing /).
  35&nbsp;echo "Creating table of contents."
  36&nbsp;
  37&nbsp;# Create an image file preparatory to burning it onto the CDR.
  38&nbsp;mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
  39&nbsp;echo "Creating ISO9660 file system image ($IMAGEFILE)."
  40&nbsp;
  41&nbsp;# Burn the CDR.
  42&nbsp;echo "Burning the disk."
  43&nbsp;echo "Please be patient, this will take a while."
  44&nbsp;wodim -v -isosize dev=$DEVICE $IMAGEFILE
  45&nbsp;#  In newer Linux distros, the "wodim" utility assumes the
  46&nbsp;#+ functionality of "cdrecord."
  47&nbsp;exitcode=$?
  48&nbsp;echo "Exit code = $exitcode"
  49&nbsp;
  50&nbsp;exit $exitcode</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="CATREF"
></A
><B
CLASS="COMMAND"
>cat</B
>, <B
CLASS="COMMAND"
>tac</B
></DT
><DD
><P
><B
CLASS="COMMAND"
>cat</B
>, an acronym for
	    <I
CLASS="WORDASWORD"
>concatenate</I
>,
	      lists a file to <TT
CLASS="FILENAME"
>stdout</TT
>. When
	      combined with redirection (<SPAN
CLASS="TOKEN"
>&#62;</SPAN
> or
	      <SPAN
CLASS="TOKEN"
>&#62;&#62;</SPAN
>), it is commonly used to concatenate
	      files.

		<A
NAME="CATUSES"
></A
>
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# Uses of 'cat'
   2&nbsp;cat filename                          # Lists the file.
   3&nbsp;
   4&nbsp;cat file.1 file.2 file.3 &#62; file.123   # Combines three files into one.</PRE
></TD
></TR
></TABLE
>

	      The <TT
CLASS="OPTION"
>-n</TT
> option to <B
CLASS="COMMAND"
>cat</B
>
	      inserts consecutive numbers before all lines of the
	      target file(s). The <TT
CLASS="OPTION"
>-b</TT
> option numbers
	      only the non-blank lines. The <TT
CLASS="OPTION"
>-v</TT
> option
	      echoes nonprintable characters, using <SPAN
CLASS="TOKEN"
>^</SPAN
>
	      notation. The <TT
CLASS="OPTION"
>-s</TT
> option squeezes multiple
	      consecutive blank lines into a single blank line.</P
><P
>See also <A
HREF="textproc.html#LNUM"
>Example 16-28</A
> and <A
HREF="textproc.html#ROT13"
>Example 16-24</A
>.</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
><A
NAME="CATLESSEFF"
></A
>
	    In a <A
HREF="special-chars.html#PIPEREF"
>pipe</A
>, it may be
	    more efficient to <A
HREF="io-redirection.html#IOREDIRREF"
>redirect</A
>
	    the <TT
CLASS="FILENAME"
>stdin</TT
> to a file, rather than to
            <B
CLASS="COMMAND"
>cat</B
> the file.
            </P
><P
>	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cat filename | tr a-z A-Z
   2&nbsp;
   3&nbsp;tr a-z A-Z &#60; filename   #  Same effect, but starts one less process,
   4&nbsp;                        #+ and also dispenses with the pipe.</PRE
></TD
></TR
></TABLE
>
            </P
></TD
></TR
></TABLE
></DIV
><P
><B
CLASS="COMMAND"
>tac</B
>, is the inverse of
	      <I
CLASS="WORDASWORD"
>cat</I
>, listing a file backwards from its end.</P
></DD
><DT
><A
NAME="REVREF"
></A
><B
CLASS="COMMAND"
>rev</B
></DT
><DD
><P
>reverses each line of a file, and outputs to
	      <TT
CLASS="FILENAME"
>stdout</TT
>.  This does not have the same effect
	      as <B
CLASS="COMMAND"
>tac</B
>, as it preserves the order of
	      the lines, but flips each one around (mirror image).</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>cat file1.txt</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>This is line 1.
 This is line 2.</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>tac file1.txt</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>This is line 2.
 This is line 1.</TT
>
 
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>rev file1.txt</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>.1 enil si sihT
 .2 enil si sihT</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="CPREF"
></A
><B
CLASS="COMMAND"
>cp</B
></DT
><DD
><P
>This is the file copy command. <TT
CLASS="USERINPUT"
><B
>cp file1
	      file2</B
></TT
> copies <TT
CLASS="FILENAME"
>file1</TT
>
	      to <TT
CLASS="FILENAME"
>file2</TT
>, overwriting
	      <TT
CLASS="FILENAME"
>file2</TT
> if it already exists (see <A
HREF="moreadv.html#EX42"
>Example 16-6</A
>).</P
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Particularly useful are the <TT
CLASS="OPTION"
>-a</TT
>
	       archive flag (for copying an entire directory tree),
	       the <TT
CLASS="OPTION"
>-u</TT
> update flag (which prevents
	       overwriting identically-named newer files), and the
	       <TT
CLASS="OPTION"
>-r</TT
> and <TT
CLASS="OPTION"
>-R</TT
> recursive
	       flags.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;cp -u source_dir/* dest_dir
   2&nbsp;#  "Synchronize" dest_dir to source_dir
   3&nbsp;#+  by copying over all newer and not previously existing files.</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="MVREF"
></A
><B
CLASS="COMMAND"
>mv</B
></DT
><DD
><P
>This is the file <I
CLASS="FIRSTTERM"
>move</I
> command.
	      It is equivalent to a combination of <B
CLASS="COMMAND"
>cp</B
>
	      and <B
CLASS="COMMAND"
>rm</B
>. It may be used to move multiple
	      files to a directory, or even to rename a directory. For
	      some examples of using <B
CLASS="COMMAND"
>mv</B
> in a script,
	      see <A
HREF="parameter-substitution.html#RFE"
>Example 10-11</A
> and <A
HREF="contributed-scripts.html#RN"
>Example A-2</A
>.</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
>When used in a non-interactive script,
	      <B
CLASS="COMMAND"
>mv</B
> takes the <TT
CLASS="OPTION"
>-f</TT
>
	      (<I
CLASS="FIRSTTERM"
>force</I
>) option to bypass user
	      input.</P
><P
>When a directory is moved to a preexisting directory,
	      it becomes a subdirectory of the destination directory.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>mv source_directory target_directory</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -lF target_directory</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>total 1
 drwxrwxr-x    2 bozo  bozo      1024 May 28 19:20 source_directory/</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="RMREF"
></A
><B
CLASS="COMMAND"
>rm</B
></DT
><DD
><P
>Delete (remove) a file or files. The <TT
CLASS="OPTION"
>-f</TT
>
	      option forces removal of even readonly files, and is useful
	      for bypassing user input in a script.</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
><A
NAME="DASHREM"
></A
></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="AEN10228"
HREF="#FTN.AEN10228"
>[2]</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 15-14</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
	       <I
CLASS="FIRSTTERM"
>ext2/ext3</I
> filesystems.</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 
	      overwritten with binary zeroes.
		<A
NAME="AEN10301"
HREF="#FTN.AEN10301"
>[3]</A
>
	      </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="AEN10336"
></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 16-2. 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
><P
>There have been various attempts at
	      <SPAN
CLASS="QUOTE"
>"automating"</SPAN
> the writing of <I
CLASS="FIRSTTERM"
>man
	      pages</I
>. For a script that makes a tentative first
	      step in that direction, see <A
HREF="contributed-scripts.html#MANED"
>Example A-39</A
>.</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.AEN10025"
HREF="external.html#AEN10025"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <TT
CLASS="OPTION"
>-v</TT
> option also orders the
                sort by <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>upper- and lowercase prefixed</I
></SPAN
>
                filenames.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN10228"
HREF="external.html#AEN10228"
>[2]</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
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN10301"
HREF="external.html#AEN10301"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>This particular feature may not yet be
		implemented in the version of the ext2/ext3 filesystem
		installed on your system. Check the documentation for
		your Linux distro.</P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
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SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
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><A
HREF="internal.html"
ACCESSKEY="P"
>Prev</A
></TD
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><TR
><TD
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><TD
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