Category Archives: Programming

Material related to computer programming.

Error handling in bash

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I’ve been learning about approaches to error handling in bash. I found Error handling in BASH, Exit Shell Script Based on Process Exit Code, Writing Robust Bash Shell Scripts and Bash: Error handling all of which contained a nugget of useful information.

Particularly I learned about $LINENO, set -e (equiv: set -o errexit), set -o pipefail, set -E, set -u (equiv: set -o nounset), set -C (equiv: set -o noclobber), #!/bin/bash -eEu, trap, shopt -s expand_aliases, alias, $PIPESTATUS, subshells, unset, and more.

If you call exit without specifying a return code then $? is used, I didn’t know that.

I found this code snippet that demos a relatively race-condition free method of acquiring a lock file:

if ( set -o noclobber; echo "$$" > "$lockfile") 2> /dev/null; 
then
   trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT

   critical-section
   
   rm -f "$lockfile"
   trap - INT TERM EXIT
else
   echo "Failed to acquire lockfile: $lockfile." 
   echo "Held by $(cat $lockfile)"
fi

Unix command to format a number of bytes as a human readable value

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It took me a while, but I finally figured out how to print a number from a bash script properly formatted with commas as thousand’s separators. For those like me who weren’t in the know, the magical incantation is:

  printf "%'d" 123456

That will format 123456 as 123,456. Much easier to read when you’re dealing with large numbers such as the sizes of files in gigabytes.

So now if I could only find the Unix command that took a number of bytes and turned it into an approximate value with GB or MB suffixes.

Yo man you are Basic

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I’ve been restoring my archives — 10,000,000 files from the past 13 years — and while I did so I had a little bit of a trip down memory lane. I found some old Visual Basic 6 database programs that I wrote when I was in high school, including a phone book, a diary, a time sheeting system, and a few other miscellaneous tools. In order to get them to run I had to get my compiler out to fix a few data object dependencies (DAO5) and a few hardcoded configuration settings such as the paths to database files. So I reinstalled my old VB6 development environment, which I’m pleased to say still works (although I’m still running Windows XP). Good fun. :)

Auto-extracting archives

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I have a directory structure with archived files, many of which are zipped or tarballed up. So if I want to search for a file, I can’t really be sure that the file I’m looking for isn’t in a compressed file. So I wrote some scripts to automatically run over a directory tree and automatically extract any compressed files that it finds there. There are a few extra features, such as if the file is larger than 10MB then it will prompt for whether to extract it or not. I have a few other features for handling errors to add in, but I’m happy to post this version up now.

extract-archives.sh

#!/bin/bash
err=$1
err=${err:-1}
search() {
	find -iname "*$1" -print0 | xargs -i -0 `dirname "$0"`/extract-file.sh "$1" "$2" "$err" "{}"
}
search ".tar.gz" "tar xf" 
search ".tgz" "tar xf" 
search ".zip" "unzip -q"

extract-file.sh

#!/bin/bash
exec 0< /dev/tty
path="$4"
file_name=`basename "$path"`
dir_name=`dirname "$path"`
new_name=`basename "$path" "$1"`
new_path="$dir_name/$new_name"
[ -e "$new_path" ] && exit 0
echo $dir_name/$file_name
file_size=`stat -c %s "$path"`
#echo -n "File is $file_size bytes."
printf "File is %'d bytes." $file_size
check=`echo "$file_size < 10485760" | bc`
if [ "$check" = "1" ]; then answer="y"; fi
while [ "$answer" != "y" ]; do
	read -n 1 -s -p " Extract? " answer
	[ "$answer" = "n" ] && echo && echo && exit 0
done
echo
mkdir "$new_path"
pushd "$new_path" > /dev/null 2>&1
$2 "../$file_name"
if [ "$?" != "0" ]; then
	popd > /dev/null 2>&1
	rm -rf "$new_path"
	echo
	answer=""
	while [ "$answer" != "y" ]; do
  		read -n 1 -s -p "Do you want to ignore this file? " answer
		[ "$answer" = "n" ] && exit $3
	done
fi
popd > /dev/null 2>&1
echo

Enabling Windows SDK 7.1 in Visual Studio 2008

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I was having a problem where Visual Studio was saying that “windows.h” could not be found after having installed the Windows SDK v7.1. I found this article which said to update the path in “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Windows\CurrentInstallFolder” in the registry. So I did that, and now everything is working.

Operator overloading in C++

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I did my first operator overloading in C++ today. My code ended up being quite different to the example code that I found, but I think I’m happy with it:

class FileData {
public:
  FileData( const string& path );
  static bool LessThan( const FileData& a, const FileData& b );
  bool operator< ( const FileData& );
  unsigned long size() const { return m_size; }
private:
  unsigned long m_size;
  char m_md5[ 16 ];
};

static bool operator< ( const FileData& a, const FileData& b ) {
  return FileData::LessThan( a, b );
}

bool FileData::operator< ( const FileData& b ) {
  return FileData::LessThan( *this, b );
}

bool FileData::LessThan ( const FileData& a, const FileData& b ) {
  unsigned long a_size = a.size();
  unsigned long b_size = b.size();
  if ( a_size == b_size ) {
    return memcmp( a.m_md5, b.m_md5, 16 ) < 0;
  }
  return a_size < b_size;
}

I'm not sure when it becomes either useful or necessary to define the operator both as a static and as a member function. I did both because I wasn't sure what was required.