# Dissect a bad oneliner ``` bash $ 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!** ``` bash $ ls *.zip | while read i; do ...; done ``` (Please read .) 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: ``` bash $ 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! ``` bash 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(r)-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 $i`". Echoing an *unquoted* variable means [wordsplitting](/syntax/expansion/wordsplit.md) will take place, so any whitespace in `$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](/syntax/expansion/cmdsubst.md) `$()` construct instead of the old style backticks: ``` bash 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](/syntax/pe.md#substring_removal): ``` bash bash $ for i in *.zip; do j="${i%.zip}"; mkdir $j; cd $j; unzip ../$i; cd ..; done ``` Let's keep going: ``` bash $ mkdir $j; cd $j; ...; cd .. ``` As a programmer, you **never** know the situation under which your program will run. Even if you do, the following best practice will never hurt: When a following command depends on the success of a previous command(s), check for success! You can do this with the "`&&`" conjunction, that way, if the previous command fails, bash will not try to execute the following command(s). It's fully POSIX(r). Oh, and remember what I said about [wordsplitting](/syntax/expansion/wordsplit.md) in the previous step? Well, if you don't quote `$j`, wordsplitting can happen again. ``` bash $ 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: ``` bash $ 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: ``` bash sh $ for i in *.zip; do j=$(basename "$i" ".zip"); mkdir "$j" && cd "$j" && unzip ../$i && cd -; done ``` ``` bash 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: ``` bash 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: ``` bash $ mkdir "$j" && unzip -d "$j" "$i" ``` ``` bash sh $ for i in *.zip; do j=$(basename "$i" ".zip"); mkdir "$j" && unzip -d "$j" "$i"; done ``` ``` bash bash $ for i in *.zip; do j="${i%.zip}"; mkdir "$j" && unzip -d "$j" "$i"; done ``` There! That's as good as it gets.