Shell Script error: "head: invalid trailing option -- 1" - linux

Question

I have this code in my shell(bash) script for splitting a file into smaller parts:
for (( i=$start; i<=$lineCount; i=i+$interval))
do
temp=`expr $i + $interval`;
if [ $temp -le $lineCount ]
then
command="head -$temp $fileName | tail -$interval > $tmpFileName";
echo "Creating Temp File: $command";
else
lastLines=`expr $lineCount - $i`;
command="head -$temp $fileName | tail -$lastLines > tmpFileName";
echo "Creating Temp File: $command";
fi
`$command`;
done
It prints the following output on stdin:
Creating Temp File: head -10 tmp.txt | tail -10 > tmp.txt_TMP
head: invalid trailing option -- 1
Try `head --help' for more information.
But the command printed: head -10 tmp.txt | tail -10 > tmp.txt_TMP runs correctly on the command line.
What am I doing wrong?

Answer 1

When you put the pipe | in a variable, the shell interprets it as an ordinary character and not as a pipe. Ditto for redirection operators like >, <, ...
An ugly way would be to use eval.
A better approach would be to split your command into different parts so as to get rid of pipes and redirection operators in it.
For example:
command="head -$temp $fileName | tail -$lastLines > tmpFileName";
would be written as:
cmd1="head -$temp $fileName";
cmd2="tail -$lastLines";
and executed by saying:
"$cmd1" | "$cmd2" > tmpFileName;
Moreover, you don't need backticks to execute a command that is stored in a variable. Simply say:
$command

Answer 2

Problem is here:
command="head -$temp $fileName | tail -$interval > $tmpFileName"
and later:
`$command`
Instead of storing whole piped command in a string you can directly execute the command:
head -$temp "$fileName" | tail -$interval > "$tmpFileName"

Answer 3

There are several issues here. First of all, lets see our refactored version with correct quoting and many other improvements:
for (( i=start; i<=lineCount; i+=interval)); do
((temp = i + interval))
if (( temp <= lineCount )); then
echo "Creating Temp File using 'tail -n $interval'"
head -n "$temp" "$fileName" | tail -n "$interval" > "$tmpFileName"
else
((lastLines = lineCount - i))
echo "Creating Temp File using 'tail -n $lastLines'"
head -n "$temp" "$fileName" | tail -n "$lastLines" > "$tmpFileName"
fi
done
I have changed all arithmetic expressions to correct syntax. This is what you want, because it is more readable.
Then, it seems like you want to put a command into a variable and then run it. To cut the story short, you simply should not do that. Here's why
Also, this is not c++, you don't have to place ; on every line.
You have also missed $ character in tmpFileName on the 10th line of your code.