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How do I know if a variable is set in Bash?
For example, how do I check if the user gave the first parameter to a function?
function a { # if $1 is set ? } 
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To check if a variable is set in Bash Scripting, use-v var or-z ${var} as an expression with if command. This checking of whether a variable is already set or not, is helpful when you have multiple script files, and the functionality of a script file depends on the variables set in the previously run scripts, etc.
'-v' or '-z' option is used to check the variable is set or unset. The above Boolean expression will return true if the variable is set and returns false if the variable is not set or empty. Parameter substitute is another way to check the variable is set or unset.
In both cases, the -z flag is a parameter to the bash's "test" built-in (a built-in is a command that is built-into the shell, it is not an external command). The -z flag causes test to check whether a string is empty. Returns true if the string is empty, false if it contains something.
if [ -z ${var+x} ]; then echo "var is unset"; else echo "var is set to '$var'"; fi where ${var+x} is a parameter expansion which evaluates to nothing if var is unset, and substitutes the string x otherwise.
Quotes can be omitted (so we can say ${var+x} instead of "${var+x}") because this syntax & usage guarantees this will only expand to something that does not require quotes (since it either expands to x (which contains no word breaks so it needs no quotes), or to nothing (which results in [ -z ], which conveniently evaluates to the same value (true) that [ -z "" ] does as well)).
However, while quotes can be safely omitted, and it was not immediately obvious to all (it wasn't even apparent to the first author of this quotes explanation who is also a major Bash coder), it would sometimes be better to write the solution with quotes as [ -z "${var+x}" ], at the very small possible cost of an O(1) speed penalty. The first author also added this as a comment next to the code using this solution giving the URL to this answer, which now also includes the explanation for why the quotes can be safely omitted.
if [ -z "$var" ]; then echo "var is blank"; else echo "var is set to '$var'"; fi This is often wrong because it doesn't distinguish between a variable that is unset and a variable that is set to the empty string. That is to say, if var='', then the above solution will output "var is blank".
The distinction between unset and "set to the empty string" is essential in situations where the user has to specify an extension, or additional list of properties, and that not specifying them defaults to a non-empty value, whereas specifying the empty string should make the script use an empty extension or list of additional properties.
The distinction may not be essential in every scenario though. In those cases [ -z "$var" ] will be just fine.
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To check for non-null/non-zero string variable, i.e. if set, use
if [ -n "$1" ] It's the opposite of -z. I find myself using -n more than -z.
You would use it like:
if [ -n "$1" ]; then echo "You supplied the first parameter!" else echo "First parameter not supplied." fi If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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