In-line text replacement using sed, shell, or some other means - bash

Question

I want to pass two parameters to a program, a file name and a modified version of the file name. The situation is I have a bunch of .html.erb files in a directory tree, and I want invoke html2haml on them with the original filename and a new output filename with the haml extension, like so:
html2haml thing.html.erb thing.html.haml
Here's my current best attempt at this:
find . -name "*.html.erb" -exec echo {} `echo {} | sed "s/.erb/.haml/g"` \;
(after I'm done testing I'll replace echo with html2haml and run it again)
However it doesn't work. The result of the expression inside backticks is the unmodified string.
Here are some experiments I tried which DO behave as expected (to test if my syntax and levels of escaping/quotes were correct):
1. echo myfile.foo | sed 's/foo/foo2/g'
2. find . -name "*.html.erb" -exec echo {} `echo xyz | sed "s/y/Y/g"` \;
3. find . -name "*.html.erb" -exec echo {} `echo {} hello` \;
4. find . -name "*.html.erb" -exec echo {} `echo {}` \;
The fact that these all behave as expected suggest to me that I am getting some small thing wrong in the syntax, and that is is indeed possible to do this with a one-liner.
If this is impossible, it might be because of a misunderstanding about "when" find inserts its results on each invocation. example #3 above suggest to me that it does it exactly when i need/expect it to (because I'm successfully concatenating each individual result string with "hello").

Answer 1

If you have gsed:
find . -name \*.erb -print0 | gsed -z 'p;s/.erb$/.haml/' | xargs -0 -n2 html2haml
If you don't have gsed and only have sed, this will work, but only if none of your file names have whitespace.
find . -name \*.erb -print | sed 'p;s/.erb$/.haml/' | xargs -n2 html2haml
Discussion about these and other techniques follows:
I have different versions of sed - my GNU sed is called gsed, if your sed is GNU - instead of gsed use sed.
You can check your sed with the sed --version, if prints something like:
sed (GNU sed) 4.2.2
Copyright (C) 2012 Free Software Foundation, Inc.
You have a GNU sed.
The above - for the next find
$ find . -name \*foo -print
./a/test.foo
./b/c/test.foo
./b/te st.foo #<- note the filename with space
./b/test.foo
the above command produces:
$find . -name \*foo -print0 | gsed -z 'p;s/foo$/foo2/' | xargs -0 -n2 echo bar
bar ./a/test.foo ./a/test.foo2
bar ./b/c/test.foo ./b/c/test.foo2
bar ./b/te st.foo ./b/te st.foo2
bar ./b/test.foo ./b/test.foo2
Without additional scripts or functions. ;)
or you can replace the sed with perl, so the next
find . -name \*foo -print0 | perl -n0le 'print;s/foo/foo2/;print' | xargs -0 -n2 echo bar
produces the same result:
bar ./a/test.foo ./a/test.foo2
bar ./b/c/test.foo ./b/c/test.foo2
bar ./b/te st.foo ./b/te st.foo2
bar ./b/test.foo ./b/test.foo2
IF you REALLY want to do it within one find, try:
find . -name \*html.erb -exec sh -c 'echo html2haml "{}" "$(echo "{}" | sed 's/\.erb/\.haml/')"' \;
or elimitating two useless echo the final command:
find . -name \*html.erb -exec sh -c 'html2haml "{}" "$(sed 's/\.erb/\.haml/'<<<"{}")"' \;

Answer 2

What about a loop?
find . -name "*.html.erb" | while read file
do
haml_file=${file%.erb}.haml
html2haml $file $haml_file
done
The ${var%glob} syntax takes an environment variable ${var} and filters out the smallest portion of the right side that matches glob.

Answer 3

If you know that the filename ends with .foo, then you can use:
do_something "$filename" "${filename%.foo}.foo2"
(In the unlikely case that you really want to just put a 2 on the end, you could of course just use "${filename}2". But I assume the foo and foo2 are to be substituted with less similar strings.)
If you want to invoke do_something from find, your best bet would be to pass it only one filename (or, better, a number of filenames each of them representing a single operation). For example:
-- do_something.sh
#!/bin/bash
# This is the definition of what you want to do.
# It is called as `bar old_filename new_filename`
bar() {
# For example
mv "$1" "$2"
}
for filename in "$#"; do
bar "$filename" "${filename%.foo}.foo2"
done
-- find command:
find . -type f -name '*.foo' -exec do_something.sh {} +
If you really need to use sed (for something that you can't even do with the bash replace syntax, ${var/pattern/substitution}), then set up do_something as above, but replace the line inside the for loop with, for example:
bar "$filename" "$(sed -r 's/([^.]+)\.([^.]+)$/\2.\1/' <<<"$filename")"
Explanation: The above sed expression (gnu-specific) flips the last two extensions around, so it would change some.file.html.en into some.file.en.html. -r causes gnu sed to use extended regex format, which I find more readable. <<< is a bashism which expands the word following it and feeds it into stdin, somewhat similar to echo "$filename" | sed ... but without creating another subprocess.

Answer 4

You can call your find like this:
find . -name "*.html.erb" -print0 -print0|xargs -0 -J % html2haml % | sed 's/\.erb$/.haml/'
This will result in executing:
html2haml thing.html.erb thing.html.haml