Shell Programming and Scripting

Hi danmero,

Could you explain me the difference, please?

I run both commands in my shell and the result is the same. I know the shell interpretation of both types of quotes, but not in this case.

Thank you.

Code:

# find . -type d | sed 's/\(.*\/.*\)[0-9]\{2\}-[0-9]\{2\}-[0-9]\{4\}\(.*\)/mv & \1$(date +%m-%d-%Y)\2/'
.
mv ./test-01-01-2010-directory ./test-$(date +%m-%d-%Y)-directory

# find . -type d | sed "s/\(.*\/.*\)[0-9]\{2\}-[0-9]\{2\}-[0-9]\{4\}\(.*\)/mv & \1$(date +%m-%d-%Y)\2/"
.
mv ./test-01-01-2010-directory ./test-10-12-2010-directory

Quote from: The Open Group Base Specifications Issue 6

They both give the same result because they both perform the same command substitution and the substituted text does not contain any sh metacharacters.. The only difference is which sh instance handles it. In the original version, the sh subshell at the end of the pipeline will handle it. In danmero's, the sh parsing the pipeline handles it.

While I am not a fan of this solution's approach (it fails if a filename that does not match the sed pattern is present, if a matching filename contains a shell metacharacter, whitespace/IFS characters), if i had to choose one I'd go with the original version because it presents better "compartmentalization"; command execution, both mv and the date command substitution are handled by the same sh.

That said, I prefer danmero's for loop approach best of all. If the parameter expansions were quoted, it'd be bulletproofed against all legal filenames. With all the other approaches, this is not possible. Also, despite the "/bin/bash", it should work fine in any posix-compliant shell.

Regards,
Alister