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1 is default; you can do
echo "$i does not exist!" >&2

The redirect to the standard is

echo "Error" 1>&2

so

#!/bin/bash
if [ $# -lt 1 ] ; then #Check there is enough command line parameters.
echo "Not enough command line parameters" 1>&2...

In bash, the piped processes run in sub-shells. So, the second exit 1 is only affecting the sub-shell running the while loop and not your whole script.

Try a variant such as:
...
while read i;...

It is due to the fact that you are using a pipeline.

To see the difference, change your code to:

#!/bin/bash

if [ $# -lt 1 ] ; then #Check there is enough command line parameters.
...

Will this work for you?

Content of program:
awk '{print $2" " $1}'

then you execute it like..

./program < input.in > output

This will output as..

cat output
b a

Change the contents of program from:
echo $2 $1to:read a1 a2 junk; echo $a2 $a1or, if the contents of input.in could contain characters with special meaning to the shell (including backslash (\) or...

Quite simple: "$1" (2, 3, ...) are variables which are filled with COMMANDLINE ARGUMENTS, whereas what you are redirecting to/from is INPUT and OUTPUT respectively. These are simply two different...

Yes, but beware if you have a large number of files that match.
The ls solution will fail with an "argument list is to long" error. The for solution is immune to this issue.

You could also use bash extend globbing:

bash$ shopt -s extglob
bash$ ls
AB AC AD AE B C
bash$ ls !(AB*|?)
AC AD AE

The shell can also do this directly without calling find or ls:

for FILE in */*.c
do
[ -f "$FILE" ] || continue
echo "$FILE"
: process the file(s)
done

Also, if you are only...

Both approaches are right.

You can also use ls command to list .c files in all sub-directories:
ls -1 */*.c

Your understanding is correct for the first part, but
[!A]?* A[!B]*
Are two patterns that produce one field list, so it is not the command that glues them together. These fields are input to the...

Ow yes, that will also not list files that start with "A" . This should work better:
ls [!A]?* A[!B]*

awk '{print "(t.who = \x27" $1 "\x27 and trunc(ymddate) = to_date(\x27" $7"-"$8"-"$NF"\x27,\x27mon-dd-yyyy\x27)) or"}' file