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copied!<p>Long ago, when all we had was <code>sh</code>, booleans where handled by relying on a convention of the <code>test</code> program where <code>test</code> returns a false exit status if run with no arguments. This allows one to think of a variable that is unset as false and variable set to any value as true. Today, test is builtin to <code>bash</code> and is commonly known by its one character alias <code>[</code> (or an executable to use in shells lacking it, as dolmen notes):</p> <pre><code>FLAG="up or <set>" if [ "$FLAG" ] ; then echo 'Is true' else echo 'Is false' fi # unset FLAG # also works FLAG= if [ "$FLAG" ] ; then echo 'Continues true' else echo 'Turned false' fi </code></pre> <p>Because of quoting conventions, script writers prefer to use the compound command <code>[[</code> that mimics <code>test</code> but has nicer syntax: variables with spaces do not need to be quoted, one can use <code>&&</code> and <code>||</code> as logical operators with weird precedence, and there are no POSIX limitations on the number of terms.</p> <p>For example, to determine if FLAG is set and COUNT is a number greater than 1:</p> <pre><code>FLAG="u p" COUNT=3 if [[ $FLAG && $COUNT -gt '1' ]] ; then echo 'Flag up, count bigger than 1' else echo 'Nope' fi </code></pre> <p><a href="https://stackoverflow.com/a/3870055/2267285">This stuff can get confusing</a> when spaces, zero length strings, and null variables are all needed and also when your script needs to work with several shells.</p>

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