Given the following code:
Max=
if [[ something exists.. ]]; then
Max=2
// .. more code that can changes the value of Max
fi
// HERE
How can I check at "HERE" if Max is equal to some number (setted)?
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Charles Duffy
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Software_t
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2 Answers
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Cool !
Max is set by default to an empty value, that when used in an arithmetic context, returns zero. For example:
Try running echo $(( definitelyNotAGivenName)) it comes out as zero, cool!
You can use the round brackets for arithmetic comparing - see more here and here
AlphaBeta
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I don't know which shell you are using but `bash` does definitely not default vars to zero. – keithpjolley Jul 05 '18 at 16:35
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@keithpjolley, in an arithmetic context it does, as an empty string (the result of expanding an unset variable) is treated as having a zero value in such a context (when unquoted -- `$(( $empty ))` evaluates to 0, `$(( "$empty" ))` is an error). – Charles Duffy Jul 05 '18 at 16:39
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Fixed the phrasing for ya – AlphaBeta Jul 05 '18 at 16:39
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Neat! I didn't know that. – keithpjolley Jul 05 '18 at 16:42
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if [ -z "$Max" ]
then
echo "Max: not set"
else if [ $Max -eq some_number ]
then
echo "Max is equal to some number"
else
echo "Max is set but not equal to some number"
fi
or
if [ -n "$Max" -a "$Max" = "some_number" ]
...
Notice that the second example is doing a string comparison which can solve some headaches but may hurt the sensibilities of purists.
keithpjolley
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Note that `-a` is marked obsolescent in the POSIX test standard; see the `OB` markers in http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html – Charles Duffy Jul 05 '18 at 16:40
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Also, `=` is a string comparator, perhaps you want `-eq`. Is it okay for `5` to be different from `05`? – ghoti Jul 05 '18 at 16:42
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@ghoti - `=` is on purpose to guard against something like `Max=five`. I assumed `some_number` is hardcoded into the script, not user input. – keithpjolley Jul 05 '18 at 16:48
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Sometimes it matters -- `test` uses that allow logical ANDs, ORs and parenthesis to control order-of-operations are obsolescent for good reason. Consider if `a='('` and `b=')'`; is `[ "$a" = "$b" ]` asking if `(` and `)` are the same character (false!), or is it a grouping operation testing whether `=` is a non-empty string (true!)? That particular corner case is a simple one, for ease of explanation, but nastier variants exist. `[[ ]]` doesn't have the problem -- being syntax, it knows which words were expanded from variables and which were part of the expression. – Charles Duffy Jul 05 '18 at 16:48