From: Brian Foster (blf at domain utvinternet.ie)
Date: Fri 05 Apr 2002 - 15:04:14 IST
| From: Stephen_Reilly at domain dell.com
| Date: Fri, 5 Apr 2002 07:33:52 -0600
|[ ... ]
| $ sed -e '1a\
| > line1\
| > line2' /etc/pam.d/su | cat > /etc/pam.d/su
| With the append option. It's unfortunate to use cat but otherwise the file
| will overwrite itself. A gold star for anyone who can remove it without
| writing to another file ... sed output redirection has often caused me
| headaches in the past. Except with certain emulaters that just run things
| sequentially ...
I would _never_ use the above!
the file is truncated to zero-length (due to the zap)
before (on most systems) sed ever even has a chance
to open it, much less read all of the contents.
if you want to modify the file in-place --- note my
sed solution did NOT try to do this! --- then sed(1)
is simply the wrong tool. use ed(1):
$ ed file
<gibberish from ed>
<more gibberish from ed>
this is still not ideal, as there is still a race with
any concurrent readers (or, worse, writers).
b.t.w., the only time I've ever had problems with
output redirection (from sed or anything else) is when
I inadvertently assume it's not buffered (including
assuming stdout and stderr appear in program order).
-- Innovative, very experienced, Unix and | Brian Foster Dublin, Ireland Chorus (embedded RTOS) kernel internals | e-mail: blf at domain utvinternet.ie expert looking for a new position ... | mobile: (+353 or 0)86 854 9268 For a resume, contact me, or see my website http://www.blf.utvinternet.ie
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