writing a command transcript to file

Question

Here is a script from Mercurial issue 2743. Call it mq.sh. I'm trying to get it to do the following things, on Debian squeeze.

1. Echo commands + standard output + standard error to a file.

2. Prefix with working directory. Sticking in $PWD doesn't work, as it presumably just keeps using the value from when it was first
   sourced.

3. Reset redirection back to normal standard output/error at the end. I'm not sure how to do this, but I imagine I'd have to use exec again.

4. I'm calling this with sh mq.sh. I'd like to do everything portably (if possible), and it seems exec is bash-specific, so I'm puzzled why this is working. Also set -x xtrace is probably bash specific as well. I got it from the Bash reference manual. Doesn't calling with sh run everything through dash on Debian?

1 currently works, I think. How do I do 2 and 3? And can I have a clarification on 4?

export PS4="$PWD$ "
exec > mq.log 2>&1
export HG="hg-crew"
rm -r main clone

set -o xtrace
$HG init main
cd main
$HG init --mq
$HG qnew foo
echo a > a
$HG add
$HG qref
$HG ci --mq -Am.
cd ..
$HG qclone main clone
cd clone
$HG paths --mq
mv .hg/patches/.hg/hgrc{,.foo}
$HG paths --mq
$HG push --mq
echo b >> a
$HG qref
$HG ci --mq -m.
mv .hg/patches/.hg/hgrc.foo .hg/patches/.hg/hgrc
$HG push --mq

Answer 1

exec for file descriptors and set -x are portable; xtrace is not, but should actually be the same thing as -x.

To save and restore file descriptors, you do indeed use exec again. You need to pick some unused descriptors, which usually means anything over 2, and redirect the originals to those, then redirect back at the end.

exec 20<&0 21>&1 22>&2
# do your redirections
# do whatever you wanted to have redirected
exec <&20 >&21 2>&22 20<&- 21>&- 22>&-

The last line first dup()s the saved file descriptors back where they came from, then closes (&-) the copies. The closes are probably unnecessary but a good idea.

Also note that, if this is being run in its own shell, the redirections go away when the shell does. You would only need to undo the redirection if you were using . (portable) or source (bash).

Answer 2

For the working directory, try PS4='+$PWD '. The single quotes mean it only gets expanded when it's about to be printed. The first character of PS4 can be printed multiple times, so you probably still want the + or at least a space at the start.

According to POSIX definition of set, set -x and set -o xtrace are both supported. The dash man page says it's OK too.