Bash script to extract HTML comment into a Markdown file

Question

I learned Bash a million years ago. I just wrote this simple script used to get the first lot of HTML comments from a file, and spit it out in order to create a README.md file.

It is just. So. Ugly. I read bits and pieces over the years, and I am sure it can be improved so much...

Here we go:

#!/bin/bash

IFS=''
active='0';
cat  hot-form-validator.html | while read "line";do

  echo $line | grep '\-\->' > /dev/null
  if [ $active = '1' -a $? = '0' ];then 
    exit 0;
  fi;

  suppress=0;
  echo $line | grep '^ *@' > /dev/null
  if [ $? = '0' ];then suppress='1'; fi;


  if [ $active = '1' -a $suppress = '0' ];then echo $line;fi;

  echo $line | grep "<!--" > /dev/null
  if [ $? = '0' ];then active='1'; fi;

done

Questions:

• Is there a better way to do grep and then check $?? Back in the day it was the way to go, but...

• Should active be a proper number rather than a string with a number? I know, it could be anything... but having a string that can be 0 or 1 just feels wrong.

• Is there a better way to preserve spaces, rather than zapping IFS?

• Any more pearls of wisdom, other than quitting my (short lived) career of bash scripter?

Answer 1

A bug

• If your html contains a backspace character e.g

  <p>The special character \n is a way to include read line</p>

your code interprets it a

The special character n is a way to include read line.

to avoid this, add "-r" switch to your read line

cat hot-form-validator.html | while read -r "line";do

• You want to check if your file exists before reading it

if [ -f $filename ]; then
#do something
else
#do something
fi

Answer 2

A Bash loop is not the best approach. If you are doing a lot of line-by-line processing, and invoking grep a lot, then awk would be a more appropriate tool.

awk '/<!--/ { ACTIVE = 1; next }
     /-->/  { exit }
     ACTIVE { print }' < hot-form-validator.html

But line-by-line processing is not an appropriate way to parse HTML. xsltproc, for example, could do a proper job of extracting the first comment in an HTML file:

xsltproc --html first-comment.xsl hot-form-validator.html

… where first-comment.xml contains:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="text"/>
  <xsl:template match="/"><xsl:value-of select="//comment()[1]"/></xsl:template>
</xsl:stylesheet>

Answer 3

Note that you can get a bunch of Bash tips instantly by pasting your code on http://www.shellcheck.net/

A couple of things jump in the eye:

• Too many unnecessary ; at the end of lines
• Instead of grep ... > /dev/null, it would be shorter to write grep -q ...
• Too many statements on a line when it would be more readable if split to multiple lines
• Instead of cat file | while ..., it would be better to write while ... < file
• Instead of echo ... | grep ..., it would be better to use here-strings, grep ... <<< ...
• Instead of grep ... and then checking the value of $? in an if, you could move the grep inside the if
• Quoting where it's not needed
• Not quoting where it's needed

With the above points improved:

#!/bin/bash

IFS=
active=0
while read "line"; do
  if [ $active = 1 ] && grep -q '\-\->' <<< "$line"; then 
    exit 0
  fi

  suppress=0
  grep -q '^ *@' <<< "$line" && suppress=1

  [ $active = 1 -a $suppress = 0 ] && echo $line

  grep -q "<!--" <<< "$line" && active=1    
done < hot-form-validator.html

I rewrote using grep -q ... <<< ... only for the sake of an example of using grep inside if statements. But a better solution is to use native Bash pattern matching with [[ ... ]], for example instead of:

  grep -q '^ *@' <<< "$line" && suppress=1

This is better, as it doesn't spawn a grep process:

  [[ "$line" =~ ^\ *@ ]] && suppress=1