Jeg ønsker at tjekke, om en variabel er sat - og hvis ikke sætter det til noget

dvs. variabelnavnet test
Jeg vil gerne tjekke om $ test er indstillet
så hvis der er noget, der er imod dette i øjeblikket - så sæt den til 0


Hvad er den bedste / korteste måde at gøre dette i et script?

Initialiserer variablen til en vis værdi sige 'x'.

var \u003d "x"

if (var ne "x") (variablen indeholder en anden værdi end 'x')
else (variablen er ikke ændret)

Brug ksh. ksh har Shell indlejret er at kontrollere, om disse betingelser.

Fra mand ksh under Parameter underudgiftsområde.

       Modifiers can be applied to the ${name} form of parameter substitution:

       ${name:-word}
              if  name  is set and not null, it is substituted, otherwise word
              is substituted.

       ${name:+word}
              if name is set and not  null,  word  is  substituted,  otherwise
              nothing is substituted.

       ${name:=word}
              if  name is set and not null, it is substituted, otherwise it is
              assigned word and the resulting value of name is substituted.

       ${name:?word}
              if name is set and not null, it is substituted,  otherwise  word
              is  printed  on  standard error (preceded by name:) and an error
              occurs (normally causing termination of a shell script, function
              or  .-script).  If word is omitted the string ?parameter null or
              not set? is used instead.

       In the above modifiers, the : can be omitted, in which case the  condi-
       tions  only  depend on name being set (as opposed to set and not null).
       If word is needed, parameter, command, arithmetic and  tilde  substitu-
       tion are performed on it; if word is not needed, it is not evaluated.

VAR=${VAR:=default_value}

[ "$VAR" ] || VAR=default_value