Bash [ -z hello ](duckduckgo.com)

174 pointsby ca98am7912 anni fa61 comments

https://duckduckgo.com/?q=bash+%5B+-z+hello+%5D

9 comments

chx12 anni fa

And yet, the top search result is tldp. This is what #bash on freenode has to say about that site:

> The infamous "Advanced" Bash Scripting Guide should be avoided unless you know how to filter out the junk. It will teach you to write bugs, not scripts. In that light, the BashGuide was written: http://mywiki.wooledge.org/BashGuide

Now, that site is on the second "page" of the search results.

mhurron12 anni fa

Do they have examples of how it's bad as opposed to simply declaring it bad?

js212 anni fa

Well for example, I just skimmed through it and I don't see an explanation of quoting, probably one of the first things I would want to explain, around the time variables are explained. It also has examples like this:

        #!/bin/bash
        for i in $( ls ); do
            echo item: $i
        done

which both uselessly calls ls (you may as well just use a glob) and doesn't work as intended if there are filenames with spaces in the directory.

You also have:

        #!/bin/bash
        for i in `seq 1 10`;
        do
                echo $i
        done

Why is it using backticks here instead of $() as in thr former example? Also, no mention that seq is not available on some systems. And, you should be using bash's sequence expression[1] anyway (added in bash 4.x), so the guide is likely out of date.

1. http://www.gnu.org/software/bash/manual/bashref.html#Brace-E...

dspillett12 anni fa

Using ls instead of a glob can be valid if you need to force ordering for some reason (you want to process things in date order for instance, IIRC the order out of a glob wildcard is alphanumeric in most places and arbitrary in some). Your point about spaces in filenames is definitely valid though but my preferred solution to that would be to stab people with a needle every time they put a space in a filename!

icebraining12 anni fa

Bah, spaces are perfectly cromulent characters. It's the bourne shell that's broken. rc works fine with them.

dspillett12 anni fa

There are a great many things on the command line (less elsewhere, but still some) that don't handle spaces in filenames well.

You are probably right that this is a technological fault rather than a problem with spaces themselves, but IMnsHO until we are not regularly using spaces as list item separators using them with the items being listed is asking for issues.

js212 anni fa

You can easily deal with spaces in bourne shell. Quote your variables. Change the IFS to make it only newlines. Of course, newlines in filenames are perfectly valid too.

thegeomaster12 anni fa

Spaces are perfectly okay in a filename. Not that I use them, but this isn't 1990 anymore.

chalst12 anni fa

I use some systems without seq, and they have Bash with version 2.x or 3.x on them (eg., OSX). seq has the nice property that writing a shell or executable replacement is easy.

I think the 2nd example is fine. It might be cut and pasted from a POSIX example.

js212 anni fa

Those are fair points. Another nit I just noticed in the second example is the extraneous semicolon.

dTal12 anni fa

>you should be using bash's sequence expression anyway

I'd love to, but it inexplicably doesn't support variables. You have to hard code the range and step. Pretty much useless for most of my use cases.

voltagex_12 anni fa

I'd like that too.

At this stage it's no better than a meme: https://groups.google.com/d/msg/comp.os.linux.misc/w0bcKoEye...

I know that if you mention the TLDP stuff in #bash you get yelled at, though.

skywhopper12 anni fa

The TLDP bash guide is full of a range of submitted examples that run the gamut of good and bad practices. That said, it's still one of the better resources out there, and I have the reference page bookmarked. But this is Bash we're talking about, "best" practice is a relative term.

zhemao12 anni fa

To be fair, searching it in Google also gives TLDP pages as the top results. I think TLDP just has better SEO.

n0rm12 anni fa

It just has more nodes pointing to its links.

That's the danger behind PageRank. If something false is popular enough, it will replace fact.

PhasmaFelis12 anni fa

Is there any alternative, though? You can't algorithmically determine truth.

r00fus12 anni fa

Not without sentiment analysis applied to links. However, that's game-able.

pavs12 anni fa

Not just more nodes, then it would have been easy, but more "credible and relevant" nodes. At least that's the idea.

Alupis12 anni fa

correct. simple back-links no longer have the weight they used to (and in some situations can actually harm your SEO).

Backlinks must come from other reputable (in google's algorithm's eye's) sites before they carry any weight.

So, this would mean, tldp.org has a lot of top-ranked linux/bash and related sites pointing to it's pages.

fyrabanks12 anni fa

I learned bash using tldp years ago, never had any major issues.

the_french12 anni fa

It seems like DDG is positioning itself to become WolframAlpha for tech. Hopefully, this will allow it to better catch the exact market segment that would be wary of search engine tracking (educated tech-savvy people).

JoshTriplett12 anni fa

Nice! It's too bad that this doesn't actually evaluate the expression in a sandbox, but nonetheless it's a very handy syntax guide.

It'd be nice to see option descriptions extracted from manpages: searching for "df -h" could then produce:

       -h, --human-readable
              print sizes in human readable format (e.g., 1K 234M 2G)

jmakeig12 anni fa

http://explainshell.com is what you're looking for.

JoshTriplett12 anni fa

I'm aware of it; I'd love to see that functionality integrated into DDG.

idank12 anni fa

Actually one of their guys approached me last time explainshell appeared on HN. I think he suggested I implement it as some sort of shortcut, but I haven't had time to look into it.

I have a branch somewhere that exposes the results as JSON, which should make it easy to integrate if they were interested.

voltagex_12 anni fa

I wonder if the developer of explainshell is reading. It looks like development has stalled. It's a really nice idea and could serve as a powerful backend for DDG.

idank12 anni fa

I am reading this!

Admittedly I haven't had a lot of time recently to improve explainshell. But I haven't deserted it; it's just a matter of finding a block of free days to focus on it.

(and as always, patches are welcome ;)

[deleted]12 anni fa

(1)

Alupis12 anni fa

What a great resource! I did not know about this before today!

frabcus12 anni fa

I note that queries like https://duckduckgo.com/?q=man+df put in a box from LinuxCommand.org

Nyubis12 anni fa

This is pretty awesome, but I don't know if people often phrase their queries like that, and I'm not sure whether it's worth it to start phrasing it like this. It doesn't seem to support more complex bash queries, so I'm not really sure how often this will be used.

mattdanger12 anni fa

[ is part of Bash syntax

embolalia12 anni fa

Nooo! Mistake #0 about shell scripting is thinking [ is syntax. It makes it way more weird and inexplicable. [ is a command, just like any other. That's why there needs to be a space between it and the next thing, and why you have to use weird looking flags and such. This is why I prefer the command `test`. It does the same thing as `[`, but is more obviously a command. And when you see it as a command, it's obvious that any command can be used in an if directly; if is just checking the exit code. (Whenever I see someone doing `if [ $? -ne 0 ]`, it makes me cry.)

jsmeaton12 anni fa

I'll bite. What should we be using instead of `if [ $? -ne 0 ]`? Because that's pretty standard in my (admittedly very basic) bash.

schoen12 anni fa

Instead of

somecommand

if [ $? -ne 0 ]; then echo "it failed"; fi

the parent commenter presumably wants to see

if ! somecommand; then echo "it failed"; fi

viraptor12 anni fa

Or:

    somecommand || echo "it failed"

if it's just one statement. There are some cases where it just looks more natural. For example:

    start_service || log "already started"

[deleted]12 anni fa

JetSpiegel12 anni fa

> when you see it as a command, it's obvious that any command can be used in an if directly; if is just checking the exit code

One of the best days of my life

manicdee12 anni fa

test is [

kl4m12 anni fa

Indeed, "test" is exactly like "[" except that it doesn't check that its last argument is a "]".

vacri12 anni fa

For some reason I've always been amused by "which [". The idea of 'punctuation' being a command or a filename is something that threw me.

frou_dh12 anni fa

...and perhaps surprisingly also a program sitting on disk.

    $ type [
    [ is a shell builtin
    $ [ 1 -gt 0
    bash: [: missing `]'

    $ whereis [
    /bin/[
    $ /bin/[ 1 -gt 0
    # success

mappu12 anni fa

You got a root shell out of `/bin/[` ? Impressive :)

e12e12 anni fa

Hm, which version of [ is that? I get:

    $ /usr/bin/[ 1 -gt 0 && echo success
    /usr/bin/[: missing `]'
    $ /usr/bin/[ 1 -gt 0 ] && echo success
    success

(This is from coreutils 8.13-3.5 on Debian GNU/Linux)

frou_dh12 anni fa

What ships in OS X 10.9, presumably BSD-derived.

[deleted]12 anni fa

wicket12 anni fa

There should a dollar sign before hello, e.g.:

bash [ -z $hello ]

...or is that a bashism that I'm not aware of? If so, that's just horrible.

Personally, I prefer to write portable shell scripts. Entering "!posix test" into DuckDuckGo to obtain the man page for "test" yields far more meaningful information for the same problem.

simias12 anni fa

[ -z hello ] is perfectly fine, but hello is not a variable, it's the string "hello". It will always return false. ddg doesn't seem to evaluate the command, just explicit it.

wicket12 anni fa

> It will always return false.

Exactly! It's not the best of examples really, is it?

imacomputer212 anni fa

"results to true if the length of 'hello' is zero." I'm glad they cleared that up. Otherwise, I might have assumed that there was no case where 'hello' had a length of zero. Joking aside, it is a useful syntax guide.

est12 anni fa

Damn, should draw FSA fors RegExps.

Like this

https://www.debuggex.com/r/IC3q9sEdkKjhuLWU

And explains it like this

http://regex101.com/

d0m12 anni fa

As an aside, I think the new design is perfect. Good job on iterating on the feedback!

pornel12 anni fa

More please! e.g.

     bash $?

is not explained.

jagtalon12 anni fa

If you're interested, you can add that in to https://github.com/duckduckgo/zeroclickinfo-goodies/blob/mas... since it's open source.

Crisco12 anni fa

I've been reading through some of those goodies, and they all seem pretty cool, but I'm wondering how secure using the 4 word random passphrase generator[0] would be...

[0] https://github.com/duckduckgo/zeroclickinfo-goodies/blob/mas...

mikeash12 anni fa

Not at all secure if you run it on DDG, since you give your password to a third party before you even get it yourself.

rsl712 anni fa

But of course you could run it ten times and pick one of them. Or modify the instant answer to return ten or twenty. Not ideal or optimal, but there it is.

mikeash12 anni fa

That hardly helps at all. Now instead of knowing your exact password, your attacker knows that your password is one of these 10-20 entries, and it's easy to just try them all.

rsl712 anni fa

Nobody has ever suggested this is a secure or ideal way to do it..

mikeash12 anni fa

My point is that it's not really even better.

vacri12 anni fa

I'm always finding another bash magic variable. '$?' is 'exit code of last command'. Last week I found '$-', which tells you which flags are currently set (set -x, set -e type of thing).

FreeFull12 anni fa

They are all listed in the man page, thankfully.