Skip to content

# 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 get the filename without its `.zip` extension. In fact, there is a POSIX®-compliant command to do this: `basename` The implementation here is suboptimal in several ways, but the only thing that's genuinely error-prone with this is "`echo $i`". Echoing an unquoted variable means wordsplitting will take place, so any whitespace in `$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 the 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 `$()` construct instead of the old style 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: When a following command depends on the success of a previous command(s), check for success! You can do this with the "`&&`" conjunction, that way, 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 word splitting, 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.

November 7, 2024
November 7, 2024

## Comments