[SOLVED] CAT giving unexpected results

danielbmartin · 03-21-2013, 10:29 AM

Have: a character matrix such as ...

Code:

abcde
fghij
klmno
pqrst

Have: a bash variable, a character string such as ...

Code:

NewRow='NEWROW'

Want: to attach the string as a new row at the top of the matrix.
This code ...

Code:

echo; echo; echo "Add a row to the top of a matrix."
echo "Input ..."; cat $InFile
echo; echo "Result ..."
NewRow='NEWROW'
 cat <<< $NewRow \
|cat - $InFile

... produces this result ...

Code:

Add a row to the top of a matrix.
Input ...
abcde
fghij
klmno
pqrst

Result ...
NEWROW
abcde
fghij
klmno
pqrst

... as expected. All good.

Now, a variation on the theme.....

Have: the same matrix and the same character string.

This code (which appears to be the inverse of that shown above) ...

Code:

echo "Add a row to the bottom of a matrix."
echo "Input ..."; cat $InFile
echo; echo "Result ..."
NewRow='NEWROW'
 cat $InFile  \
|cat - <<< $NewRow

... produces this unexpected result ...

Code:

Add a row to the bottom of a matrix.
Input ...
abcde
fghij
klmno
pqrst

Result ...
NEWROW

Please explain.

Note 1) There are other ways to "tack on" a row to the top or bottom of a matrix, such as sed.
I'm not looking for alternate solutions, but only an explanation for why cat failed.

Note 2) Please don't chide me for a "useless" cat. That's not the point of the question.

Daniel B. Martin

grail · 03-21-2013, 11:00 AM

Its late so have not thought this all the way through, however, the issue is not cat but rather your use of re-directions and which order they are being processed in.

I'll have another look in the morning in case no one else elaborates or you don't find the solution using my information

ntubski · 03-21-2013, 11:54 AM

Quote:

Originally Posted by danielbmartin

Code:

 cat $InFile  \
|cat - <<< $NewRow

There is only a single stdin stream, you are trying to connect it to 2 things. The same problem as:

Code:

cat < file1 < file2

PTrenholme · 03-21-2013, 12:10 PM

And, to get the result you want, with minimal modification to your logic, use

Code:

echo; echo; echo "Add a row to the top of a matrix."
echo "Input ..."; cat $InFile
echo; echo "Result ..."
NewRow='NEWROW'
 tac <<< $NewRow | tac - $InFile | tac

mina86 · 03-21-2013, 12:47 PM

Uh… What's wrong with:

Code:

printf '%s\n' "$NewRow" | cat - "$InFile"

Also, remember to quote your variables. If you aren't sure if you have to, quote.

danielbmartin · 03-21-2013, 02:45 PM

Quote:

Originally Posted by mina86

Uh… What's wrong with:

Code:

printf '%s\n' "$NewRow" | cat - "$InFile"

Your code plants the NewRow at the top of the matrix, not the bottom.

Repeating from the original post:

Quote:

I'm not looking for alternate solutions, but only an explanation for why cat failed.

danielbmartin · 03-21-2013, 02:52 PM

Quote:

Originally Posted by PTrenholme

And, to get the result you want, with minimal modification to your logic, use

Code:

echo; echo; echo "Add a row to the top of a matrix."
echo "Input ..."; cat $InFile
echo; echo "Result ..."
NewRow='NEWROW'
 tac <<< $NewRow | tac - $InFile | tac

Your code works but I hesitate to use tac three times when one sed does the same job.

Repeating from the original post:

Quote:

I'm not looking for alternate solutions, but only an explanation for why cat failed.

Daniel B. Martin

colucix · 03-21-2013, 04:03 PM

Indeed, as ntubski explained, the problem is the double redirection of the standard input from the pipe and from the here string. The reason is that the last one supersedes the preceding one. In this case the cat command doesn't receive input from the pipe anymore, but only from the here string.

ntubski · 03-21-2013, 06:53 PM

Quote:

Originally Posted by danielbmartin

Your code plants the NewRow at the top of the matrix, not the bottom.

Oh, I missed that you were trying to do that too.

Quote:

Note 2) Please don't chide me for a "useless" cat. That's not the point of the question.

Without the distraction of the redundant cat it's easier to see what the correct inverse is:

Code:

# stdin ("NEWROW") then $InFile
cat - "$InFile" <<< "$NewRow"
# $InFile then stdin
cat "$InFile" - <<< "$NewRow"

danielbmartin · 03-21-2013, 08:56 PM

Quote:

Originally Posted by ntubski

Without the distraction of the redundant cat it's easier to see what the correct inverse is:

Code:

# stdin ("NEWROW") then $InFile
cat - "$InFile" <<< "$NewRow"
# $InFile then stdin
cat "$InFile" - <<< "$NewRow"

Excellent! Thank you for these crisp and instructive gems!

Thank you also to everyone who contributed to this thread.

SOLVED

Daniel B. Martin

danielbmartin · 03-21-2013, 09:33 PM

Quote:

Originally Posted by mina86

... remember to quote your variables. If you aren't sure if you have to, quote.

Thank you, this is good advice.
Misplaced or missing quotes have caused many a stumble.

Daniel B. Martin