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Dissect a bad oneliner

$ ls *.zip | while read i; do j=`echo $i | sed 's/.zip//g'`; mkdir $j; cd $j; unzip ../$i; cd ..; done

This is an actual one-liner someone asked about in #bash. There are several things wrong with it. Let's break it down!

$ ls *.zip | while read i; do ...; done

(Please read http://mywiki.wooledge.org/ParsingLs.) This command executes ls on the expansion of *.zip. Assuming there are filenames in the current directory that end in '.zip', ls will give a human-readable list of those names. The output of ls is not for parsing. But in sh and bash alike we can loop safely over the glob itself:

$ for i in *.zip; do j=`echo $i | sed 's/.zip//g'`; mkdir $j; cd $j; unzip ../$i; cd ..; done

Let's break it down some more!

j=`echo $i | sed 's/.zip//g'` # where $i is some name ending in '.zip'

The goal here seems to be just get the filename without its .zip extension. In fact, there is a POSIX®-compliant command to do this already: basename! The implementation here is suboptimal in several ways, but the only thing that's genuinely error-prone with this is "echo $i". Echoing an unquoted value means that wordsplitting will take place, so any whitespace in $i will essentially be normalized. In sh it is necessary to use an external command and a subshell to achieve the goal, but we can eliminate a pipe (subshells, external commands, and pipes carry extra overhead when they launch, so they can really hurt performance in a loop). Just for good measure, let's use the more readable, modern $() instead of oldschool backticks:

sh $ for i in *.zip; do j=$(basename "$i" ".zip"); mkdir $j; cd $j; unzip ../$i; cd ..; done

In Bash we don't need the subshell or the external basename command. See Substring removal with parameter expansion:

bash $ for i in *.zip; do j="${i%.zip}"; mkdir $j; cd $j; unzip ../$i; cd ..; done

Let's keep going:

$ mkdir $j; cd $j; ...; cd ..

As a programmer, you never know the situation under which your program will run. Even if you do, the following best practice will never hurt: Whenever your following command depends on the success of your previous command(s), check for success! You can do this with the "&&" conjunction, which means that if the previous command fails, bash will not try to execute the following command(s). It's fully POSIX®. Oh, and remember what I said about wordsplitting in the previous step? Well, if you don't quote $j, wordsplitting can happen again.

$ mkdir "$j" && cd "$j" && ... && cd ..

That's almost right, but there's one problem – what happens if $j contains a slash? Then cd .. will not return to the original directory. That's wrong! cd - causes cd to return to the previous working directory, so it's a much better choice:

$ mkdir "$j" && cd "$j" && ... && cd -

(If it occurred to you that I forgot to check for success after cd -, good job! You could do this with { cd - || break; }, but I'm going to leave that out because it's verbose and I think it's likely that we will be able to get back to our original working directory without a problem.)

So now we have:

sh $ for i in *.zip; do j=$(basename "$i" ".zip"); mkdir "$j" && cd "$j" && unzip ../$i && cd -; done

bash $ for i in *.zip; do j="${i%.zip}"; mkdir "$j" && cd "$j" && unzip ../$i && cd -; done

Let's throw the unzip command back in the mix:

mkdir "$j" && cd "$j" && unzip ../$i && cd -

Well, besides the wordsplitting, there's nothing terribly wrong with this. Still, did it occur to you that unzip might already be able to target a directory? There isn't a standard for the unzip command, but all the implementations I've seen can do it with the -d flag. So we can drop the cd commands entirely:

$ mkdir "$j" && unzip -d "$j" "$i"

sh $ for i in *.zip; do j=$(basename "$i" ".zip"); mkdir "$j" && unzip -d "$j" "$i"; done

bash $ for i in *.zip; do j="${i%.zip}"; mkdir "$j" && unzip -d "$j" "$i"; done

There! That's as good as it gets.

Discussion

Michael Shigorin, 2012/01/11 11:55

Thanks, nice walkthrough :) I'd only stress proper quoting a bit more, not introducing any unquoted variable expansions myself.

Eduardo Bustamante, 2012/06/18 03:04

I'd like to address some issues I noticed in the dissection.

First, in the section that states: ``but the only thing that's genuinely error-prone with this is "echo $i".. I think ``sed 's/.zipg''' is also error prone. Let me explain it. The dot is a RE meta-character, which matches *anything*. So, it will match things like azip, bzip & czip. Also, if the goal is to strip the extension, it will require some anchoring (i.e. s/\.zip$); or else, it will remove more than just the extension. If that anchor is used, there's no need for the `g' flag. I'm not stating that using sed is the way to go; I'm merely remarking on its usage in that one-liner. Also, echo has multiple incompatible implementations, and using it to print an arbitrary string is risky, since that string can take the form of an option (-e or -n for example). There's no way to avoid this, like using `–', since echo will just print it. Its replacements are printf (printf '%s\n' "$i") or using the here-string syntax in bash («< "$i"). The next thing to note is in the differentiation between sh and bash regarding the ${i%.zip} expansion. The ${name%foo} expansion is standardized in POSIX, so it's safe to use it in sh also. And it's clearly simpler, since you can do just ${i%.*}. And the last thing. It can be made to work with other casings of .zip, like .Zip, .ZIP, and all the possible permutations, using *.[Zz][Ii][Pp] as the pattern, or just using shopt -s nocaseglob.

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howto/dissectabadoneliner.txt · Last modified: 2009/04/09 06:22 (external edit)
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