问题
Say I read some data into a Bash array:
$ IFS=" " read -a arr <<< "hello/how are/you iam/fine/yeah"
Now, I want to print the first /-sliced field for each element in the array.
What I do is to loop over the elements and use shell parameter expansion to strip everything from the first /:
$ for w in "${arr[@]}"; do echo "${w%%/*}"; done
hello
are
iam
However, since printf allows us to print the whole content of the array in a single expression:
$ printf "%s\n" "${arr[@]}"
hello/how
are/you
iam/fine
... I wonder if there is a way to use the shell parameter expansion ${w%%/*} at the time of using printf, instead of looping over all the elements and doing it against every single one.
回答1:
Oh, I just found the way: just use the parameter expansion normally, only that against ${arr[@]} instead of ${arr}!
$ IFS=" " read -a arr <<< "hello/how are/you iam/fine/yeah"
$ printf "%s\n" "${arr[@]%%/*}"
hello
are
iam
Greg's wiki helped here:
Parameter Expansion on Arrays
BASH arrays are remarkably flexible, because they are well integrated with the other shell expansions. Any parameter expansion that can be carried out on a scalar or individual array element can equally apply to an entire array or the set of positional parameters such that all members are expanded at once, possibly with an additional operation mapped across each element.
$ a=(alpha beta gamma) # assign to our base array via compound assignment $ echo "${a[@]#a}" # chop 'a' from the beginning of every member lpha beta gamma $ echo "${a[@]%a}" # from the end alph bet gamm $ echo "${a[@]//a/f}" # substitution flphf betf gfmmf
来源：https://stackoverflow.com/questions/37698108/shell-parameter-expansion-on-arrays