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J. Doe
Asked: 2018-07-04 14:03:38 +0800 CST

How to get sed to work for parametric expansions?

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The current script is

for r in $path/[0-9]*.tst; do

sed -i s/val1/${r%.tst}/ $path/foo.bar
sed -i s/val2/${var2}/ $path/foo.bar
sed -i s/val3/${var3}/ $path/foo.bar
sed -i s/val4/${var4}/ $path/foo.bar

cat $path/foo.bar
done

Values 2-4 work fine; however, val1 remains unchanged. This is true even when I use ${r} instead of ${r%.tst}.

Are there any glaring errors in this snippet?

bash sed

1 Answers

  1. The main problem is that $r will look like path/file.tst so when it's expanded (whether with or without the dot extension), sed will see something like

    sed s/val1/path/file/

    which will cause an error because the / in the replacement string is interpreted as the terminating / of the sed s/pattern/replacement/

    You can overcome that by choosing any other character as sed's pattern separator - choose something that's not going to be in your filename or path e.g. @ (although of course that is a legal filename character). So

    "s@val1@${r%.tst}@"

    (I've used double-quotes here: they will allow variable expansion while preventing other potential gotchas such as glob expansion).

    As a matter of style (and efficiency) I'd suggest not making multiple sed calls to edit the same file - you can chain multiple replacement expressions in GNU sed with -e e.g.

    sed -i -e "s@val1@${r%.tst}@" -e "s@val2@${var2}@" -e ...

    or make use of the -f scriptfile option and use a here-document:

    for r in "$path"/[0-9]*.tst; do

sed -i -f- "$path/foo.bar" << EOF
s@val1@${r%.tst}@
s@val2@${var2}@
s@val3@${var3}@
s@val4@${var4}@
EOF

cat "$path/foo.bar"

done
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