Diff for "BashFAQ/031"

Anchor(faq31)

What is the difference between the old and new test commands ([ and [[)?

[ ("test" command) and [[ ("new test" command) are both used to evaluate expressions. Some examples:

    if [ -z "$variable" ]
    then
        echo "variable is empty!"
    fi

    if [ ! -f "$filename" ]
    then
        echo "not a valid, existing file name: $filename"
    fi

and

    if [[ -e $file ]]
    then
        echo "directory entry does not exist: $file"
    fi

    if [[ $file0 -nt $file1 ]]
    then
        echo "file $file0 is newer than $file1"
    fi

To cut a long story short: [ implements the old, portable syntax of the command. Although all modern shells have built-in implementations, there usually still is an external executable of that name, e.g. /bin/[. [[ is a new improved version of it, which is a keyword, not a program. This has beneficial effects on the ease of use, see below. [[ is understood by KornShell, ["BASH"] (e.g. 2.03), and KornShell93, but not by the older BourneShell.

Although [ and [[ have much in common, and share many expression operators like "-f", "-s", "-n", "-z", there are some notable differences. Here is a comparison list:

Special primitives that [[ is defined to have, but [ may be lacking (depending on the implementation):

But there are more subtle differences.

• No field splitting will be done for [[ (and therefore many arguments need not be quoted)

   file="file name"
   [[ -f $file ]] && echo "$file is a file"

  will work even though $file is not quoted and contains whitespace. With [ the variable needs to be quoted:

   file="file name"
   [ -f "$file" ] && echo "$file is a file"

  This makes [[ easier to use and less error-prone.

• No file name generation will be done for [[. Therefore the following line tries to match the contents of the variable $path with the pattern /*

   [[ $path = /* ]] && echo "\$path starts with a forward slash /: $path"

  The next command most likely will result in an error, because /* is subject to file name generation:

   [ $path = /* ] && echo "this does not work"

  (If you need to do that using Bourne shells, use case instead.)

• As a rule of thumb, [[ is used for strings and files. If you want to compare numbers, use an ArithmeticExpression, e.g.

   i=0
   while ((i<10)); do ...

When should the new test command [[ be used, and when the old one [? If portability to the BourneShell is a concern, the old syntax should be used. If on the other hand the script requires ["BASH"] or KornShell, the new syntax is much more flexible.

See the [http://wooledge.org:8000/BashGuide#Bash_Tests 'Bash Tests' chapter in the Bash Guide].