waitwhat on Writing Robust Bash Shell Scripts

Comment on Writing Robust Bash Shell Scripts parent

    $ cat foo.sh
    #!/bin/bash
    echo $BASH_VERSION
    unset filename
    if [[ $filename == "foo" ]];
    $ ./foo.sh
    4.1.2(1)-release
    ./foo.sh: line 5: syntax error: unexpected end of file

If $filename evaluates to an empty string, then the last line of code becomes

    if [[ == "foo" ]];

Similarly, if $filename evaluates to a b c, then the last line of code becomes

    if [[ a b c == "foo" ]];

The bash interpreter has problems with both of these.

nescafe 14y

No, that is just wrong. In double-brackets, bash will handle expansion and testing of unset variables and variables with spaces just fine, because word splitting and pathname expansion are not performed in double braces -- they are shell syntax, as opposed to single brackets, which are commands. Your syntax error happened because you don't have a fi matching that if.

As a counterexample:

  (echo $BASH_VERSION; 
  foo=foo; [[ $foo = foo ]] && echo $foo; 
  foo="1 2 3"; [[ $foo = "1 2 3" ]] && echo $foo; 
  foo=''; echo ${foo?"No foo for you"}; 
  unset foo; echo ${foo?"No foo for you"})
  4.2.10(1)-release
  foo
  1 2 3
  
  bash: foo: No foo for you

That last one is one of the few ways you can tell whether a variable is unset or merely set to an empty string.

lrn2bash.

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