bash-hackers-wiki/docs/syntax/ccmd/conditional_expression.md

# The conditional expression

## Synopsis

    [[ <EXPRESSION> ]]

## Description

The conditional expression is meant as the modern variant of the
[classic test command](../../commands/classictest.md). Since it is **not** a
normal command, Bash doesn't need to apply the normal commandline
parsing rules like recognizing `&&` as [command
list](../../syntax/basicgrammar.md#lists) operator.

The testing features basically are the same (see the lists for [classic
test command](../../commands/classictest.md)), with some additions and
extensions.

|Operator|Description|
|--|--|
|`( <EXPRESSION> )`|Used to group expressions, to influence precedence of operators|
|`<EXPRESSION1> && <EXPRESSION2>`|`TRUE` if `<EXPRESSION1>`**and**`<EXPRESSION2>` are `TRUE` (do **not** use `-a`!)|
|`<EXPRESSION1>||<EXPRESSION2>`|`TRUE` if `<EXPRESSION1>`**or**`<EXPRESSION2>` is `TRUE` (do **not** use `-o`!)|
|`<STRING> == <PATTERN>`|`<STRING>` is checked against the pattern `<PATTERN>` - `TRUE` on a match<br>*But note¹, quoting the pattern forces a literal comparison.*|
|`<STRING> = <PATTERN>`|equivalent to the `==` operator|
|`<STRING> != <PATTERN>`|`<STRING>` is checked against the pattern `<PATTERN>` - `TRUE` on **no match**|
|`<STRING> =~ <ERE>`|`<STRING>` is checked against the [extended regular expression](https://en.wikipedia.org/wiki/Regular_expression#POSIX_extended) `<ERE>` - `TRUE` on a match|
|See the [classic test operators](../../commands/classictest.md#file_tests)|Do **not** use the `test`-typical operators `-a` and `-o` for AND and OR.|
|See also [arithmetic comparisons](../../syntax/arith_expr.md#comparisons)|Using `(( <EXPRESSION> ))`, the [arithmetic expression compound command](../../syntax/ccmd/arithmetic_eval.md)|

When the `==` and `!=` operators are used, the string to the right of
the operator is considered a pattern and matched according to the rules
of [Pattern Matching](../../syntax/pattern.md). If the shell option
`nocasematch` is enabled, the match is performed without regard to the
case of alphabetic characters.

¹Any part of the pattern may be quoted to force it to be matched as a
literal string.

When the operators `<` and `>` are used (string collation order), the
test happens using the current locale when the `compat` level is greater
than "40".

Operator precedence (highest => lowest):

-   `( <EXPRESSION> )`
-   `! <EXPRESSION>`
-   `<EXPRESSION1> && <EXPRESSION2>`
-   `<EXPRESSION1> || <EXPRESSION2>`

Do **not** use the `test`-typical operators `-a` and `-o` for AND and
OR, they are not known to the conditional expression. Instead, use the
operators `&&` and `||`.

### Word splitting

[Word splitting](../../syntax/expansion/wordsplit.md) and [pathname
expansion](../../syntax/expansion/globs.md) are not performed in the expression
you give. That means, a variable containing spaces can be used without
quoting:

    sentence="Be liberal in what you accept, and conservative in what you send"
    checkme="Be liberal in what you accept, and conservative in what you send"
    if [[ $sentence == $checkme ]]; then
      echo "Matched...!"
    else
      echo "Sorry, no match :-("
    fi

Compare that to the [classic test command](../../commands/classictest.md), where
word splitting is done (because it's a normal command, not something
special):

    sentence="Be liberal in what you accept, and conservative in what you send"
    checkme="Be liberal in what you accept, and conservative in what you send"
    if [ "$sentence" == "$checkme" ]; then
      echo "Matched...!"
    else
      echo "Sorry, no match :-("
    fi

You need to quote that variable reference in the classic test command,
since (due to the spaces) the word splitting will break it otherwise!

### Regular Expression Matching

Using the operator `=~`, the left hand side operand is matched against
the **extended regular expression (ERE)** on the right hand side.

This is consistent with matching against patterns: Every quoted part of
the regular expression is taken literally, even if it contains regular
expression special characters.

Best practise is to put the regular expression to match against into a
variable. This is to avoid shell parsing errors on otherwise valid
regular expressions.

    REGEX="^[[:upper:]]{2}[[:lower:]]*$"

    # Test 1
    STRING=Hello
    if [[ $STRING =~ $REGEX ]]; then
      echo "Match."
    else
      echo "No match."
    fi
    # ==> "No match."

    # Test 2
    STRING=HEllo
    if [[ $STRING =~ $REGEX ]]; then
      echo "Match."
    else
      echo "No match."
    fi
    # ==> "Match."

The interpretation of quoted regular expression special characters can
be influenced by setting the `compat31` and `compat32` shell options
(`compat*` in general). See [shell_options](../../internals/shell_options.md).

#### The special BASH_REMATCH array variable

An array variable whose members are assigned by the `=~` binary operator
to the `[[` conditional command.

The element with index 0 is the portion of the string matching the
entire regular expression. The element with index n is the portion of
the string matching the nth parenthesized subexpression.

See [BASH_REMATCH](../../syntax/shellvars.md#bash_rematch).

Example:

    if [[ "The quick, red fox" =~ ^The\ (.*),\ (.*)\ fox$ ]]; then
        echo "${BASH_REMATCH[0]} is ${BASH_REMATCH[1]} and ${BASH_REMATCH[2]}.";
    fi

    ==> The quick, red fox is quick and red.

### Behaviour differences compared to the builtin test command

As of Bash 4.1 alpha, the test primaries '&lt;' and '&gt;' (compare
strings lexicographically) use the current locale settings, while the
same primitives for the builtin test command don't. This leads to the
following situation where they behave differently:

    $ ./cond.sh
    [[ ' 4' < '1' ]]        --> exit 1
    [[ 'step+' < 'step-' ]] --> exit 1
    [ ' 4' < '1' ]         --> exit 0
    [ 'step+' < 'step-' ]  --> exit 0

It won't be aligned. The conditional expression continues to respect
the locate, as introduced with 4.1-alpha, the builtin `test`/`[` command
continues to behave differently.

### Implicit arithmetic context

When you use a numeric comparison, the arguments are evaluated as an
arithmetic expression. The arithmetic expression must be quoted if it
both contains whitespace and is not the result of an expansion.

    [[ 'i=5, i+=2' -eq 3+4 ]] && echo true # prints true.

## Examples

## Portability considerations

-   `[[ ... ]]` functionality isn't specified by POSIX(R), though it's
    a reserved word
-   Amongst the major "POSIX-shell superset languages" (for lack of a
    better term) which do have `[[`, the test expression compound
    command is one of the very most portable non-POSIX features. Aside
    from the `=~` operator, almost every major feature is consistent
    between Ksh88, Ksh93, mksh, Zsh, and Bash. Ksh93 also adds a large
    number of unique pattern matching features not supported by other
    shells including support for several different regex dialects, which
    are invoked using a different syntax from Bash's `=~`, though `=~`
    is still supported by ksh and defaults to ERE.
-   As an extension to POSIX ERE, most GNU software supports
    backreferences in ERE, including Bash. According to POSIX, only BRE
    is supposed to support them. This requires Bash to be linked against
    glibc, so it won't necessarily work on all platforms. For example,
    `$(m='(abc(def))(\1)(\2)'; [[ abcdefabcdefdef =~ $m ]]; printf '<%s> ' $? "${BASH_REMATCH[@]}" )`
    will give `<0> <abcdefabcdefdef> <abcdef> <def> <abcdef> <def>`.
-   the `=~` (regex) operator was introduced in Bash 3.0, and its
    behaviour changed in Bash 3.2: since 3.2, quoted strings and
    substrings are matched as literals by default.
-   the behaviour of the `<` and `>` operators (string collation order)
    has changed since Bash 4.0

## See also

-   Internal: [pattern matching language](../../syntax/pattern.md)
-   Internal: [the classic test command](../../commands/classictest.md)
-   Internal: [the if-clause](../../syntax/ccmd/if_clause.md)
-   [What is the difference between test, \[ and \[\[
    ?](http://mywiki.wooledge.org/BashFAQ/031) - BashFAQ 31 - Greg's
    wiki.