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added table 5.3 which briefly demonstrates how variables can be set in the shell

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tomit4 2022-05-07 17:10:54 -07:00
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unix_programming_environment/evaluation_of_shell_variables.txt Normal file
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--------------------- Table 5.3 ---------------------
$var value of var; nothing if var is undefined
${var} same; useful if alphanumerics follows 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