Piping into a part of bash command stored in variable [duplicate] - linux

Question

This question already has answers here:
Conditional step in a pipeline
(2 answers)
Can I make a shell function in as a pipeline conditionally "disappear", without using cat?
(1 answer)
Closed 4 months ago.
EMPTY_VAR=''
MMDDYYYY='6.18.1997'
PIPE_VAR=' | xargs echo "1+" | bc'
echo "$MMDDYYYY" | cut -d "." -f 2${EMPTY_VAR}
>> 18
Command above would give me correct output, which is 18, but if I try to use PIPE_VAR instead it would give me bunch of errors:
echo "$MMDDYYYY" | cut -d "." -f 2${PIPE_VAR}
cut: '|': No such file or directory
cut: xargs: No such file or directory
cut: echo: No such file or directory
cut: '"1+"': No such file or directory
cut: '|': No such file or directory
cut: bc: No such file or directory
OR:
echo "$MMDDYYYY" | cut -d "." -f 2"$PIPE_VAR"
cut: invalid field value ‘| xargs echo "1+" | bc’
Try 'cut --help' for more information.
What I'm really trying to find out is that even possible to combine commands like this?

Answer 1

You can't put control operators like | in a variable, at least not without resorting to something like eval. Syntax parsing comes before parameter expansion when evaluating the command line, so Bash is only ever going to see that | as a literal character and not pipeline syntax. See BashParsing for more details.
Conditionally adding a pipeline is hard to do well, but having a part of the pipeline conditionally execute one command or another is more straightforward. It might look something like this:
#!/bin/bash
MMDDYYYY='6.18.1997'
echo "$MMDDYYYY" | cut -d "." -f 2 |
if some_conditional_command ; then
xargs echo "1+" | bc
else
cat
fi

Answer 2

It looks like you're trying to calculate the next day. That's hard to do with plain arithmetic, particularly with month/year ends.
Let date do the work. This is GNU date. It can't parse 6.18.1997 but it can parse 6/18/1997
for MMDDYYYY in '2.28.1996' '2.28.1997'; do
date_with_slashes=${MMDDYYYY//./\/}
next_day=$(date -d "$date_with_slashes + 1 day" '+%-m.%-d.%Y')
echo "$next_day"
done
2.29.1996
3.1.1997