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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://www.gnu.org/software/bash/manual/bash.html#index-test.

• Test if a given variable is equal/not equal to the specified string:
  [ "${{variable}}" {{==|!=}} "{{string}}" ]
• Test if a given variable is [eq]ual/[n]ot [e]qual/[g]reater [t]han/[l]ess [t]han/[g]reater than or [e]qual/[l]ess than or [e]qual to the specified number:
  [ "${{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}} ]