when using Double Square brackets and when single square brackets

 

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  • unary operator expected in shell script when comparing null value with string 2 answers
  • unary operator expected 4 answers

    • Unlike [[ ]] , single square brackets are subject to word splitting like normal commands. Think of it similarly as passing arguments to test builtin. [[ ]] are special and variables within it are not split with IFS. This is why it's preferable to use [[ ]] since it's more efficient and safer.

    This one is just one of the things you can't do with [ . With [ you'd play around with { } and ( ) to make it work. Note that ( ) summons a subshell and { } would always need a semicolon in the end which is ugly. However you can do it safely with [[ ]] : 

    [[ a != b && c != d && ( something || something ) ]]

    By the way to fix your problem you have to place the variable in double-quotes to prevent splitting: 

    [ "$STRING" = dog ]

    With [[ it's not needed.

    Sere Word Splitting: https://www.gnu.org/software/bash/manual/html_node/Word-Splitting.html

    Add:

    It seems like $STRING there was actually skipped as an argument for being empty instead of being split. Similarly the concept of it being interpreted as a normal command would apply and applying double quotes would fix it.

    Because [[ is special. [ needs to retain Bourne shell compatibility, including its limitations. Since [[ is a construct introduced separately in bash, it can be fixed.

    It's tough to search on, but this is very frequently asked. Try googling for "bash square brackets" and you will see lots of discussions on this topic, mostly on SO:

    https://www.google.com/search?q=bash+square+brackets

    https://serverfault.com/questions/52034/what-is-the-difference-between-double-and-single-square-brackets-in-bash

    http://mywiki.wooledge.org/BashGuide/TestsAndConditionals#Conditional_Blocks_.28if.2C_test_and_.5B.5B.29

    The short answer is to always use [[ unless you know otherwise...

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