the bash way is faster, but only with bash

Some bashisms are syntax sugar at first sight, such as the += concatenation syntax. Usually, they happen to be faster than their more portable counterparts. But only with bash itself.

Take the following script as an example:

#!/bin/sh
# script1-portable.sh
part="$(seq 1 100000)"

for i in $(seq 1 10); do
seq="${part}"
seq="${seq}${part}"
done

$ time bash script1-portable.sh
user 0m20.837s

Now, compare to the following script that uses += :

#!/bin/sh
# script1-bash.sh
part="$(seq 1 100000)"

for i in $(seq 1 10); do
seq="${part}"
seq+="${part}"
done

$ time bash script1-bash.sh
user 0m14.227s

Yes, it's faster. However, when the first script is run with dash:

$ time dash script1-portable.sh
user    0m0.609s

[[ is another example:

#!/bin/sh
# script2-portable.sh
a="$(seq 1 100000)"; b="$(seq 1 100)"

for i in $(seq 1 10); do
[ "$a" = "$b" ]
done

$ time bash script2-portable.sh
user    0m9.148s

And the version using the bashism:

#!/bin/sh
# script2-bash.sh
a="$(seq 1 100000)"; b="$(seq 1 100)"

for i in $(seq 1 10); do
[[ $a = $b ]]
done

$ time bash script2-bash.sh
user    0m4.223s

Then again, the bash way is faster, yet it doesn't compare to dash:

$ time dash script2-portable.sh
user    0m0.588s