Diff for "BashFAQ/031"

Differences between revisions 10 and 11

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, as shown below. [[ is understood by KornShell and BASH (e.g. 2.03), but not by the older POSIX or 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:

Feature	new test `[[`	old test `[`	Example
string comparison	>	\>	-
	<	\<	-
	= (or ==)	=	-
	!=	!=	-
expression grouping	&&	-a	`[[ -n $var && -f $var ]] && echo "$var is a file"`
expression grouping	`\|\|`	-o	-
Pattern matching	= (or ==)	(not available)	`[[ $name = a* ]] \|\| echo "name does not start with an 'a': $name"`
RegularExpression matching	=~	(not available)	`[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"`

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

Description	Primitive	Example
entry (file or directory) exists	-e	`[[ -e $config ]] && echo "config file exists: $config"`
file is newer/older than other file	-nt / -ot	`[[ $file0 -nt $file1 ]] && echo "$file0 is newer than $file1"`
two files are the same	-ef	`[[ $input -ef $output ]] && { echo "will not overwrite input file: $input"; exit 1; }`
negation	!	-

But there are more subtle differences.

No WordSplitting or glob expansion 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.

Parentheses in [[ do not need to be escaped:

 [[ -f $file1 && ( -d $dir1 || -d $dir2) ]]
 [ -f "$file1" -a \( -d "$dir1" -o -d "$dir2" \) ]

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

# Bash
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 Bash Tests chapter in the BashGuide.

-  ⇤ ← Revision 10 as of 2008-06-08 08:48:22 → 
  Size: 3549
  Editor: 59
  Comment:
+   ← Revision 11 as of 2008-11-22 14:09:00 → ⇥
  Size: 3550
  Editor: localhost
  Comment: converted to 1.6 markup
-Deletions are marked like this.
+Additions are marked like this.
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-[[Anchor(faq31)]]
+<<Anchor(faq31)>>
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-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, as shown below. {{{[[}}} is understood by KornShell and ["BASH"] (e.g. 2.03), but not by the older POSIX or BourneShell.
+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, as shown below. {{{[[}}} is understood by KornShell and [[BASH]] (e.g. 2.03), but not by the older POSIX or BourneShell.
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- * No WordSplitting or [:glob:] expansion will be done for {{{[[}}} (and therefore many arguments need not be quoted):
+ * No WordSplitting or [[glob]] expansion will be done for {{{[[}}} (and therefore many arguments need not be quoted):
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-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.
+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.
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-See the [:BashGuide/Practices/BashTests:Bash Tests] chapter in the BashGuide.
+See the [[BashGuide/Practices/BashTests|Bash Tests]] chapter in the BashGuide.