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performance bug of [[ $x ]]

From: Peng Yu
Subject: performance bug of [[ $x ]]
Date: Sat, 7 Mar 2020 16:48:46 -0600

[[ $x ]] just tests whether the variable $x is of length 0 or not. So
its performance should not depend on how long the variable is.

But the following test case shows that the run time does depend on the
length of the variable.

Should it be considered as a performance bug of bash?

$ x=$(printf '%.sx' {1..1000000})
$ time [[ $x ]]

real    0m0.004s
user    0m0.003s
sys     0m0.000s
$ x=$(printf '%.sx' {1..10000000})
$ time [[ $x ]]

real    0m0.043s
user    0m0.039s
sys     0m0.004s


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