"[ ]" vs. "[[ ]]" in Bash shell [duplicate]

Question

This may be answered already but I am going to ask it anyways. I have two versions of a script (comp.sh)-

#!/bin/sh
export tDay=$(date '+%Y%m%d')
newfile="filename_$tDay"
filename="filename_20120821100002.csv"
echo $newfile $filename
if [ $filename = *$newfile* ]
then
  echo "Matched"
else
  echo "Not Matched!"
fi

Output:
$ ./comp.sh
filename_20120821 filename_20120821100002.csv
Not Matched!

And

#!/bin/sh
export tDay=$(date '+%Y%m%d')
newfile="filename_$tDay"
filename="filename_20120821100002.csv"
echo $newfile $filename
if [[ $filename = *$newfile* ]]
then
  echo "Matched"
else
  echo "Not Matched!"
fi

$ comp.sh
filename_20120821 filename_20120821100002.csv
Matched

Could someone explain me Why the difference?

Also - under what circumstances should [ ] be used vs. [[ ]] and vice versa?

Answer 1

[[ is a bash built-in, and cannot be used in a #!/bin/sh script. You'll want to read the Conditional Commands section of the bash manual to learn the capabilities of [[. The major benefits that spring to mind:

• == and != perform pattern matching, so the right-hand side can be a glob pattern
• =~ performs regular expression matching. Captured groups are stored in the BASH_REMATCH array.
• boolean operators && and ||
• parenthèses for grouping of expressions.
• no word splitting, so it's not strictly necessary to quote your variables.

The major drawback: your script is now bash-specific.