Shell script blog -OR- Give me tips on how to write better scripts!

upsidaisium · 2011-02-22 08:14:29

Hi folks,

I don't write a lot of shell scripts, and the ones that I do write are usually quite short, so as a learning exercise I made a shell/CGI blog script.. And I was hoping that some of the shell scripting experts in the community could give me some pointers on form, style, etc! The script works just fine, but maybe I haven't coded it very elegantly. Or maybe in some places I haven't necessarily used the right (or most appropriate?) commands to accomplish what I want.

It's a simple blog. It can either:
- Display a blog post (by default the newest, unless the name of a post is given)
- Display a listing of posts for a certain tag

Posts are saved in normal files in markdown format. Tagging is accomplished by putting the name of the tag(s) into the filename, separated by hyphens. For example: the file "wonderful_blog_post-whining-computers-archlinux.md" would be a post named "wonderful blog post", and it would have three tags ("whining", "computers", "archlinux"). Maybe it's not the best idea in the world... but I went with it anyhow

I put the script on pastebin for you all to see: http://pastebin.com/L8nMpUjT

Please do take a look and let me know if I've done something badly! Hopefully I can learn a thing or two from your comments.

And there's a working example of it here: http://www.ynnhoj.net/thedump/s.cgi

freak · 2011-02-22 16:22:18

http://mywiki.wooledge.org/BashGuide
http://mywiki.wooledge.org/BashFAQ/001

I would guess that you learned from the so-called "Advanced" bash guide, which is pretty terrible to be honest.

The biggest issue I've seen is using "for" to iterate through the output of a command. As FAQ 1 (the second link) explains, this should never be done. It makes for really fragile code.

There is also a lack of quoting, and ls should never be in a script. See http://mywiki.wooledge.org/ParsingLs and http://mywiki.wooledge.org/BashPitfalls

I hope this helps... I don't mean to tear this apart or anything, but you asked for criticism, and there it is. Good luck with this, and I hope you can turn it into much more robust code using this guide.

upsidaisium · 2011-02-23 01:16:13

I don't mind criticism as long as it's constructive Like I said before, my script works but I wasn't if I had actually coded it well!

I haven't read any guides, by the way. I've just patched this together using the bit of sh knowledge that I already have and by looking at scripts (of all sorts) that other people have written to learn more.

I'll take a look at the links you've provided and try to make the changes that you've suggested. Regarding using ls, I was pretty sure that I shouldn't be using it. But what about this line:

ls -t $(find -iname '*.md' | sed 's,^\./,,')

In this context would using ls be acceptable? And if not, how can I use find (or some other command) to sort my files by the modification time?

Thanks!

Edit: Another question, if you don't mind, regarding using a while loop. This works for me:

find . -name "*.md" | sed -e 's,\./,,' | while read -r post; do
        echo $post
done

So.. would you consider this the preferred way to do it?

Last edited by upsidaisium (2011-02-23 01:52:50)

falconindy · 2011-02-23 01:58:03

To sort files by modification time (safely) requires GNU stat and some bash:

declare -a files
while IFS=$'\t' read -r -d '' ts file; do
  files+=("$file")
done < <(stat --printf '%Y\t%n\0' * | sort -zn)

printf "%s\n" "${files[@]}"

The array is of course unnecessary as you could print inside the loop, but I see no way around the usage of null delimiters and bashisms (read's -d flag in particular) to be 100% safe.

sisco311 · 2011-02-23 02:20:10

upsidaisium wrote:

Another question, if you don't mind, regarding using a while loop. This works for me:
find . -name "*.md" | sed -e 's,\./,,' | while read -r post; do
        echo $post
done
So.. would you consider this the preferred way to do it?

I think, the preferred method is:

while IFS= read -rd '' file; do
  file="${file##*/}"  # or file="${file#*/} # I'm not sure if I fully understand your code :)
  echo "$file"
done < <(find . -name '*.md' -print0)

But, please check out the links posted by freak or seek for answers on the #bash channel on freenode.

Last edited by sisco311 (2011-02-23 02:28:30)

upsidaisium · 2011-02-23 06:43:28

falconindy wrote:

To sort files by modification time (safely) requires GNU stat and some bash:
declare -a files
while IFS=$'\t' read -r -d '' ts file; do
  files+=("$file")
done < <(stat --printf '%Y\t%n\0' * | sort -zn)

printf "%s\n" "${files[@]}"
The array is of course unnecessary as you could print inside the loop, but I see no way around the usage of null delimiters and bashisms (read's -d flag in particular) to be 100% safe.

Thanks for the example, though I'm trying to avoid bash and write my script in posix shell. So after some tinkering, this is what I've come up with to make a simple list of blog post file names:

#!/bin/sh

cd posts

while IFS=$'\t' read -r ts file; do
        ALLPOSTS="$ALLPOSTS $file"
done << EOF
$(stat --printf '%Y\t%n\n' *.md | sort -rn)
EOF

echo $ALLPOSTS

So I suppose that it isn't 100% safe - but could it be acceptable? Is there anything else I could do?

Edit: as it turns out, my server is running some flavour of BSD, I think FreeBSD. So the above examples don't work with FreeBSD's stat. Ah well..

Last edited by upsidaisium (2011-02-23 08:26:29)

freak · 2011-02-23 17:56:20

It's possible to get the newest file from a directory using nothing but POSIX shell (assuming [ has -nt, think it does).

latest() {
  local file latest
  for file in "{1:-.}"/*; do
    [ "$file" -nt "$latest" ] && latest=$file
  done
  printf "%s\n" "$latest"
}
latest [dir]

Actuall sorting by mtime is a really tricky issue though... it's enough of a pain with bash and GNU coreutils as it is.

Last edited by freak (2011-02-23 17:58:32)

Arch Linux

#1 2011-02-22 08:14:29

Shell script blog -OR- Give me tips on how to write better scripts!

#2 2011-02-22 16:22:18

Re: Shell script blog -OR- Give me tips on how to write better scripts!

#3 2011-02-23 01:16:13

Re: Shell script blog -OR- Give me tips on how to write better scripts!

#4 2011-02-23 01:58:03

Re: Shell script blog -OR- Give me tips on how to write better scripts!

#5 2011-02-23 02:20:10

Re: Shell script blog -OR- Give me tips on how to write better scripts!

#6 2011-02-23 06:43:28

Re: Shell script blog -OR- Give me tips on how to write better scripts!

#7 2011-02-23 17:56:20

Re: Shell script blog -OR- Give me tips on how to write better scripts!

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