[[

Check file types and compare values. Returns a status of 0 if the condition evaluates to true, 1 if it evaluates to false. More information: https://gnu.org/software/bash/manual/bash.html#index-_005b_005b.

• Test if a given variable is equal/not equal to the specified string:

[[ $variable ==|!= "string" ]]

• Test if a given string conforms the specified glob/regex:

[[ $variable ==|=~ pattern ]]

• Test if a given variable is [eq]ual/[n]ot [e]qual/[g]reater [t]han/[l]ess [t]han/[g]reater than or [e]qual/[l]ess than or [e]qual to the specified number:

[[ $variable -eq|ne|gt|lt|ge|le integer ]]

• Test if the specified variable has a [n]on-empty value:

[[ -n $variable ]]

• Test if the specified variable has an empty value:

[[ -z $variable ]]

• Test if the specified [f]ile exists:

[[ -f path/to/file ]]

• Test if the specified [d]irectory exists:

[[ -d path/to/directory ]]

• Test if the specified file or directory [e]xists:

[[ -e path/to/file_or_directory ]]