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I want to attach a string to each of the elements in an array in bash, but not if the array is empty.

Currently this works for non-empty arrays:

$ cat this.sh
#!/bin/bash
lst=$@
lst[0]="a"
lst[1]="b"
echo ${lst[@]/%/.ext}
$ bash this.sh
a.ext b.ext

But it fails for an empty array:

$cat that.sh 
#!/bin/bash
lst=$@
echo ${lst[@]/%/.ext}
$ bash that.sh 
.ext

So this fails, as I want the string to be empty, if the array is empty.

Restriction: I can't use an if-statement or loop constructs in this particular situation.

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  • Can you use tests without if? eg [ ${#lst} -gt 0 ] && echo ${lst[@]/%/.ext} Incidentally, your lst variable isn't initially an array, just a string containing all the positional parameters. Commented May 8, 2014 at 15:07
  • @JoshJolly: no, unfortunately I can't use this. Commented May 8, 2014 at 15:11
  • How is your program supposed to do something under a certain condition if you dont test for the condition ? Are you allowed to use awk or sed ? Commented May 8, 2014 at 15:15

2 Answers 2

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2

There's a slight misunderstanding in your OP. In your second example, lst is not an empty array. Check this out: add the line declare -p lst at the end of your script (the declare builtin used with the -p option will print out the information about the given variable):

#!/bin/bash
lst=$@
declare -p lst

then run your script:

$ ./myscript
declare -- lst=""

So for Bash, your lst is just an empty string (yet it is set). Look what an empty array is:

$ a=()
$ declare -p a
declare -a a='()'

and look what happens if I run your echo command:

$ echo ${a[@]/%/.ext}

$

did you see that? it's really empty!

I want to say that it really hurt me when I wrote echo ${a[@]/%/.ext} without quotes! the proper way would be:

$ echo "${a[@]/%/.ext}"

$

(still empty, eh).

Now you probably mean:

#!/bin/bash
lst=( "$@" )
echo "${lst[@]/%/.ext}"

I called this script banana, I chmod +x banana and I played with it:

$ ./banana

$ ./banana one two
one.ext two.ext
$ ./banana "one one" two
one one.ext two.ext
$ ./banana one ""
one.ext .ext
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2

In your second case, lst isn't an empty array: it's just an empty string. Ensure that lst is an array by using

ls=( "$@" )

In your first case, lst is an array because you ignore the initial assignment to lst by overwriting it with the following indexed assignments.

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