Why do we need a closing square bracket in if-blocks?

Question

If the opening bracket is synonymous with the test command, then I have a question - why do I need a closing bracket?

if [ $a -eq $b ]

In later versions of bash, the [[...]] (reserved word) construction is supported. What are the differences?

Why does an error occur when using [...] and several conditions (&&, ||), but does not occur when using the [[...]] construct?

 if [ $a -eq $b && $c -eq $d ] - ошибка if [[ $a -eq $b && $c -eq $d ]] - не ошибка

UPD: The #! / Bin / bash interpreter is used.
Scripts format * .sh

aleksandr barakin aleksandr barakin 56.8k 13 48 172 · Accepted Answer · 2016-05-19T15:19:32

information gathered from man bash .

Let's start with the terminology.
1. [[ - this is a compound command
2. [ and test is just builtin commands (inline commands)
the compound command (their list is far from being limited to the [[ ) command is that its argument can be a compound expression combining expressions with the help of the following operators (listed in the priority order of the calculation):
1. ( выражение ) - group expressions to change the order of calculations / execution
2. ! выражение ! выражение - "not"
3. выражение1 && выражение2 - “and”
4. выражение1 || выражение2 выражение1 || выражение2 - "or"
the command [ not exactly a synonym for the test command. the syntax of their call is somewhat different, although functionally they are identical:
```
 test выражение [ выражение ] 
```
the builtin commands [ and test have similar, but slightly different syntax, operators for grouping, negation, and logical operations “and / or”:
1. \( выражение \) - grouping
2. ! выражение ! выражение - “not” (syntax is identical here)
3. выражение1 -a выражение2 - "and"
4. выражение1 -o выражение2 - "or"

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