goelweb.com --> Software --> Unix utilities --> Unix shell --> Evaluation of Shell Variables

Evaluation of Shell Variables

$var

value of var; nothing if var undefined

${var}

same; useful if alphanumerics follow variable name

${var-thing}

value of var if defined; otherwise thing. $var unchanged.

${var=thing}

value of var if defined; otherwise thing. If undefined, $var set to thing

${var?message}

if defined, $var. Otherwise print message and exit shell. If message empty, print: var: parameter not set

${var+thing}

thing if $var defined, otherwise nothing