Undocumented Bash - How to Check if a Shell Variable is Set or Not

狂写 bash 脚本数日，偶有所得。

Usually, we use [ -n "$VAR" ] to check if a shell variable contain non null string. But bash man page does not describe how to check if a shell variable is set or not.

After some search and investigation, I got the following missing bash manual

Parameter Expansion

${parameter-word}
    Use Default Values.  If parameter is unset, the expansion of word
    is substituted.  Otherwise, the value of parameter is substituted.
${parameter=word}
    Assign Default Values.  If parameter is unset, the expansion of
    word is assigned to parameter.  The value of parameter is then
    substituted.  Positional parameters and special parameters may not
    be assigned to in this way.
${parameter?word}
    Display Error if Unset.  If parameter is unset, the expansion of
    word (or a message to that effect if word is not present) is
    written to the standard error and the shell, if it is not
    interactive, exits.  Otherwise, the value of parameter is
    substituted.
${parameter+word}
    Use Alternate Value.  If parameter is unset, nothing is
    substituted, otherwise the expansion of word is substituted.

Some section in bash man page for reference

${parameter:-word}
    Use Default Values.  If parameter is unset or null, the expansion
    of word is substituted.  Otherwise, the value of parameter is
    substituted.
${parameter:=word}
    Assign Default Values.  If parameter is unset or null, the
    expansion of word is assigned to parameter.  The value of
    parameter is then substituted.  Positional parameters and special
    parameters may not be assigned to in this way.
${parameter:?word}
    Display Error if Null or Unset.  If parameter is null or unset,
    the expansion of word (or a message to that effect if word is not
    present) is written to the standard error and the shell, if it is
    not interactive, exits.  Otherwise, the value of parameter is
    substituted.
${parameter:+word}
    Use Alternate Value.  If parameter is null or unset, nothing is
    substituted, otherwise the expansion of word is substituted.

Here are some examples

[ -n "${VAR+x}" ]  ## Fails if VAR is unset
[ -n "${VAR:+x}" ] ## Fails if VAR is unset or empty
[ -n "${VAR}" ]    ## Fails if VAR is unset or empty
[ -n "${VAR-x}" ]  ## Succeeds if VAR is unset
[ -n "${VAR:-x}" ] ## Succeeds if VAR is unset or empty
[ -z "${VAR}" ]    ## Succeeds if VAR is unset or empty