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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-test.

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

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

• Test if a given variable is equal/not equal/greater than/less than/greater than or equal/less than or equal to the specified number:

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

• Test if the specified variable has a non-empty value:

[ -n "$variable" ]

• Test if the specified variable has an empty value:

[ -z "$variable" ]

• Test if the specified file exists:

[ -f path/to/file ]

• Test if the specified directory exists:

[ -d path/to/directory ]

• Test if the specified file or directory exists:

[ -e path/to/file_or_directory ]