ch22_22.htm

the unix power tools

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&#13;



<A
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HREF="ch46_05.htm"
TITLE="46.5 Stop Syntax Errors in String Tests "
>x</A
> <A
CLASS="co"
HREF="ch47_04.htm"
TITLE="47.4 C Shell Variable Operators and Expressions "
>!~</A
> 


<A
CLASS="co"
HREF="ch45_07.htm"
TITLE="45.7 The exec Command "
>exec</A
> <A
CLASS="co"
HREF="ch09_06.htm"
TITLE="9.6 String Editing (Colon) Operators "
>:q</A
> 


&#13;</PRE
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>
&#13;#!/bin/csh -f
# su - fix incorrect $HOME and USER with system 'su' command
foreach arg ($argv)
   # find first non-option argument
   if (&quot;x$arg&quot; !~ x-*) then
      setenv HOME ~$arg
      setenv USER $arg
      exec /bin/su $argv:q
   endif
end
echo &quot;$0 ERROR: can't find username.&quot;
exit 1</PRE
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></P
><P
CLASS="para"
>Another workaround for that is an alias with the name of the account I'm
<EM
CLASS="emphasis"
>su</EM
>ing to:</P
><P
CLASS="para"
><TABLE
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><TH
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><PRE
CLASS="calloutlist"
>
<A
CLASS="co"
HREF="ch13_07.htm"
TITLE="13.7 The () Subshell Operators "
>(..)</A
> </PRE
></TH
><TD
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><PRE
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>
alias randi '(setenv HOME ~randi; setenv USER randi; su randi)'</PRE
></TD
></TR
></TABLE
></P
><P
CLASS="para"
>There's another problem that can happen on any version of UNIX:
the account you <EM
CLASS="emphasis"
>su</EM
> to
<SPAN
CLASS="link"
>doesn't have permission (<A
CLASS="linkend"
HREF="ch22_02.htm"
TITLE="Tutorial on File and Directory Permissions "
>22.2</A
>)</SPAN
>
to access the current directory where you ran the <EM
CLASS="emphasis"
>su</EM
> command.
Then, you may get an error like <CODE
CLASS="literal"
>getwd: can't stat .</CODE
> from the C shell
on the account you <EM
CLASS="emphasis"
>su</EM
> to.
Or you may get no error but the <EM
CLASS="emphasis"
>su</EM
> will fail.
The

fix for both problems is to <EM
CLASS="emphasis"
>cd</EM
> to a world-access directory like
<EM
CLASS="emphasis"
>/</EM
> or <EM
CLASS="emphasis"
>/tmp</EM
> before you run <EM
CLASS="emphasis"
>su</EM
>.
An alias can make that easy:</P
><P
CLASS="para"
><TABLE
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><TR
><TH
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><PRE
CLASS="calloutlist"
>
<A
CLASS="co"
HREF="ch10_06.htm"
TITLE="10.6 Avoiding C Shell Alias Loops "
>\su</A
> </PRE
></TH
><TD
VALIGN="TOP"
><PRE
CLASS="screen"
>
alias su '(cd /; \su \!*)'</PRE
></TD
></TR
></TABLE
></P
><P
CLASS="para"
>You can also add the <CODE
CLASS="literal"
>cd /</CODE
> command to this shell script if you want.</P
><P
CLASS="para"
>If the account you <EM
CLASS="emphasis"
>su</EM
> to runs the C shell (and you don't use the
<EM
CLASS="emphasis"
>-f</EM
> option--see below), it will read the <EM
CLASS="emphasis"
>.cshrc</EM
> file.
If that <EM
CLASS="emphasis"
>.cshrc</EM
> has hardcoded pathnames or commands that only the
other account can run, the commands might fail.
That can cause a variety of &quot;fun&quot; problems.
Try replacing hardcoded pathnames like
<CODE
CLASS="literal"
>/home/</CODE
><CODE
CLASS="replaceable"
><I
>oldacct</I
></CODE
><CODE
CLASS="literal"
>/bin</CODE
> with paths that use account-specific
variables like <CODE
CLASS="literal"
>$home/bin</CODE
>, <CODE
CLASS="literal"
>~/bin</CODE
>, and so on.</P
><P
CLASS="para"
>Plain <CODE
CLASS="literal"
>su</CODE
> <CODE
CLASS="replaceable"
><I
>whoever</I
></CODE
> doesn't read a
C shell user's <EM
CLASS="emphasis"
>.login</EM
> file or a Bourne shell user's <EM
CLASS="emphasis"
>.profile</EM
>.
Using <CODE
CLASS="literal"
>su -</CODE
> <CODE
CLASS="replaceable"
><I
>whoever</I
></CODE
> (see the section &quot;Other su
Features&quot; at the end of this chapter) solves
that,
but you can't suspend an <EM
CLASS="emphasis"
>su -</EM
> shell (at least not on my systems).</P
><P
CLASS="para"
>Finally, because the <EM
CLASS="emphasis"
>su</EM
> command runs in a 
<SPAN
CLASS="link"
>subshell (<A
CLASS="linkend"
HREF="ch38_04.htm"
TITLE="Subshells "
>38.4</A
>)</SPAN
>,
<SPAN
CLASS="link"
>environment variables (<A
CLASS="linkend"
HREF="ch06_01.htm#UPT-ART-1170"
TITLE="What Environment Variables Are Good For "
>6.1</A
>)</SPAN
>
set in the account you <EM
CLASS="emphasis"
>su</EM
> from will be passed into
the subshell.
That can be good or bad.
For instance, your favorite
<SPAN
CLASS="link"
>EDITOR (<A
CLASS="linkend"
HREF="ch06_03.htm"
TITLE="Predefined Environment Variables "
>6.3</A
>)</SPAN
>
(<EM
CLASS="emphasis"
>vi</EM
>, Emacs, or whatever)
can be passed to the account you <EM
CLASS="emphasis"
>su</EM
> to.
But that account might also set a variable that you want to use.
If you're wondering what's set after you <EM
CLASS="emphasis"
>su</EM
>, type <CODE
CLASS="literal"
>set</CODE
> for a
list of shell variables, and either
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>env</EM
> or <EM
CLASS="emphasis"
>printenv</EM
> (<A
CLASS="linkend"
HREF="ch06_01.htm#UPT-ART-1170"
TITLE="What Environment Variables Are Good For "
>6.1</A
>)</SPAN
>
to see environment variables.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-1920-SECT-1.4"
>22.22.4 Quick Changes to Other Accounts </A
></H3
><P
CLASS="para"
>If your system is busy, it can take time to run through all the commands
in the other user's <EM
CLASS="emphasis"
>.cshrc</EM
> file.
The <EM
CLASS="emphasis"
>su</EM
> command can pass arguments to the subshell it starts, though.
If the other account uses C shell, the <EM
CLASS="emphasis"
>-f</EM
> option tells it not to read
the <EM
CLASS="emphasis"
>.cshrc</EM
> file (for example, <CODE
CLASS="literal"
>su -f </CODE
><CODE
CLASS="replaceable"
><I
>whoever</I
></CODE
>).
You won't get that account's <EM
CLASS="emphasis"
>.cshrc</EM
> setup, but you will start to work on
it sooner.</P
><P
CLASS="para"
>If logging in on your system takes a long time and you want to switch to
another account permanently, you can
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>exec</EM
> (<A
CLASS="linkend"
HREF="ch45_07.htm"
TITLE="The exec Command "
>45.7</A
>)</SPAN
>
the <EM
CLASS="emphasis"
>su</EM
> command:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>exec su </B
></CODE
><CODE
CLASS="replaceable"
><I
>whoever</I
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>That makes a weird situation where the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>who</EM
> (<A
CLASS="linkend"
HREF="ch51_04.htm"
TITLE="Who's On? "
>51.4</A
>)</SPAN
>
command will show you logged
on as your original account, but you'll be running as <CODE
CLASS="replaceable"
><I
>whoever</I
></CODE
>
(the <EM
CLASS="emphasis"
>whoami</EM
> or <EM
CLASS="emphasis"
>id</EM
> command will tell you that).
Also, because the <EM
CLASS="emphasis"
>su</EM
> shell isn't a
<SPAN
CLASS="link"
>login shell (<A
CLASS="linkend"
HREF="ch02_08.htm"
TITLE="Identifying Login Shells "
>2.8</A
>)</SPAN
>,
the <EM
CLASS="emphasis"
>logout</EM
> command
won't work; you'll need to type <CODE
CLASS="literal"
>exit</CODE
> instead.
So, <EM
CLASS="emphasis"
>exec su</EM
> is a little tricky-but it's fast.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-1920-SECT-1.5"
>22.22.5 Other su Features </A
></H3
><P
CLASS="para"
>The command <EM
CLASS="emphasis"
>su -e</EM
>, which may also be called <EM
CLASS="emphasis"
>su -m</EM
> or
<EM
CLASS="emphasis"
>su -p</EM
>, switches to the other user's account but keeps
the environment you have now.
That's handy when you're having trouble with the other user's environment
or want to keep your own.
(Some <EM
CLASS="emphasis"
>su</EM
>s do <EM
CLASS="emphasis"
>-m</EM
> by default, more or less.
Use
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>env</EM
> or <EM
CLASS="emphasis"
>printenv</EM
> (<A
CLASS="linkend"
HREF="ch06_01.htm#UPT-ART-1170"
TITLE="What Environment Variables Are Good For "
>6.1</A
>)</SPAN
>
to see what you get.)</P
><P
CLASS="para"
>The command <EM
CLASS="emphasis"
>su -</EM
> simulates a full login to the other account.
If the other account runs the Bourne shell, the <EM
CLASS="emphasis"
>.profile</EM
> will be read.
For the C shell, both <EM
CLASS="emphasis"
>.cshrc</EM
> and <EM
CLASS="emphasis"
>.login</EM
> will be read.
You can't <EM
CLASS="emphasis"
>suspend</EM
> a <EM
CLASS="emphasis"
>su -</EM
> subshell.
When you log out though, you'll be back in your original account's shell.<A
CLASS="indexterm"
NAME="AUTOID-25151"
></A
></P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
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