veyron on Writing Robust Bash Shell Scripts

Comment on Writing Robust Bash Shell Scripts

He suggests using

    if [ "$filename" = "foo" ];

Why not

    if [[ $filename == "foo" ]];

EDIT: aware of the fact that [[ ]] is a bash extension, but the actual title of the post is "Writing Robust Bash Shell Scripts", implying that it should be using the best stuff for BASH.

waitwhat 14y

    $ 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.

Dobbs 14y

First is Posix, the second is bash. The first is more portable.

veyron 14y

The title of the article is "Writing Robust Bash Shell Scripts", which differs from the title of the HN post.

ralph 14y

    if [[ $filename == foo ]];

Nick_C 14y

Which is the correct idiomatic way. The article's author is wrong in enclosing foo in quotes.

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