The concept of a current directory doesn't exist within Ant. You would be better off making your directory references relative to a defined property.
The immediate reason that your task failed is because "cd" wasn't locatable as an external command. Often 'cd' is built into the command shell and doesn't exist as a discrete external program.
For programs that do exist discretely, it's better to specify their full pathname. Not only does that help Ant find them, it protects you from alternative programs that have the same name - such programs may exist for several reasons, some good, some evil.
However, returning specifically to 'cd'. If 'cd' was an external command, it's not unlikely that Ant would have issued the 'cd', changed the directory for duration of that task, then reverted to its original setting. Which is one more reason why you don't want to use the 'cd' command in Ant.
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