Difference between conditional expressions using [ and [[

Question

What's the precise difference between:

if [ $? -ne 0 ];

and

if [[ $? -ne 0 ]];

`[` is (generally) the name of a program. This should help explain some of the differences. — , Jun 29 '12 at 04:31
In bash, `[` is a builtin (try `type [`), but `[` is often an external program as well. — Michael Hoffman, Jun 29 '12 at 04:34
Please see [BashFAQ/031](http://mywiki.wooledge.org/BashFAQ/031). — Dennis Williamson, Jun 29 '12 at 08:52
Also see [this](http://stackoverflow.com/questions/2188199/bash-double-or-single-bracket-parentheses-curly-braces/2188369#2188369). — Dennis Williamson, Jun 29 '12 at 08:58

score 3 · Accepted Answer · edited Jun 29 '12 at 16:23

3

As stated here:

Contrary to [, [[ prevents word splitting of variable values. So, if VAR="var with spaces", you do not need to double quote $VAR in a test - eventhough using quotes remains a good habit. Also, [[ prevents pathname expansion, so literal strings with wildcards do not try to expand to filenames. Using [[, == and != interpret strings to the right as shell glob patterns to be matched against the value to the left, for instance: [[ "value" == val* ]].

edited Jun 29 '12 at 16:23

vergenzt

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answered Jun 29 '12 at 04:28

higuaro

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A small addendum: The abilities like prevention of word splitting and path name expansion are a consequence of the fact that `[[` is a built-in as compared to `[` which is an external command. – Samveen Jun 29 '12 at 06:50
1

@Samveen: `[` is an external command. But it's also a builtin. – Dennis Williamson Jun 29 '12 at 08:50

score 2 · Answer 2 · answered Jun 29 '12 at 04:26

There is none. The [[...]] syntax introduces some other things you can do with conditional expressions, though. From help [[:

Returns a status of 0 or 1 depending on the evaluation of the conditional
expression EXPRESSION.  Expressions are composed of the same primaries used
by the `test' builtin, and may be combined using the following operators:

  ( EXPRESSION )    Returns the value of EXPRESSION
  ! EXPRESSION              True if EXPRESSION is false; else false
  EXPR1 && EXPR2    True if both EXPR1 and EXPR2 are true; else false
  EXPR1 || EXPR2    True if either EXPR1 or EXPR2 is true; else false

When the `==' and `!=' operators are used, the string to the right of
the operator is used as a pattern and pattern matching is performed.
When the `=~' operator is used, the string to the right of the operator
is matched as a regular expression.

Yes, there is absolutely no difference between the results of `[ $? -ne 0 ]` and `[[ $? -ne 0 ]]`. — Michael Hoffman, Jun 29 '12 at 13:48

Difference between conditional expressions using [ and [[

2 Answers2