Can someone explain why A and B behave differently?
A=`echo hello how are you | wc -w`
and
CMD="echo hello how are you | wc -w"
B=`$CMD`
They give different results:
$echo $A
4
$echo $B
hello how are you | wc -w
What I would like to have is a command in a variable that I can execute at several points of a script and get different values to compare. It used to work fine but if the command has a pipe, it doesn't work.
`` (i.e. backticks) or $() in bash referred as command substitution."" - used e.g. to preserves the literal value of characters, i.e. data.In the your first example, the command echo hello how are you | wc -w is executed and its value 4 assigned to A, hence you get 4.
In your second example, the assignment of a string to a variable B and by `$CMD` the | is not "evaluated" because of late word splitting (see here for further information), and you get hello how are you | wc -w.
What you need can be done with eval command as follows:
CMD="echo hello how are you | wc -w"
echo `eval $CMD` # or just eval "$CMD"
# Output is 4
$CMD its value is not "evaluated" --> if that were the case, then B would equal "echo hello how are you | wc -w" but it doesn't: it equals "hello how are you | wc -w". So the echo command was executed. It just didn't pipe into wc.
eval "$CMD", not eval $CMD. Otherwise you can get filenames injected into code if your string word-splits to include any globs.
eval-- it is a huge bug magnet).