How can I prepend a string to the beginning of each line in a file?

a one-line awk command should do the trick also:

awk '{print "prefix" $0}' file

The entire loop can be replaced by a single sed command that operates on the entire file:

sed -e 's/^/prefix/' $file

Concerning your original error:

./appendToFile.sh: line 11: /bin/sed: Argument list too long

The problem is with this line of code:

sed -e 's/^/prefix/' $line

$line in this context is file name that sed is running against. To correct your code you should fix this line as such:

echo $line | sed -e 's/^/prefix/'

(Also note that your original code should not have the < $file at the end.)

William Pursell addresses this issue correctly in both of his suggestions.

However, I believe you have correctly identified that there is an issue with your original text file. dos2unix will not correct this issue, as it only strips the carriage returns Windows sticks on the end of lines. (However, if you are attempting to read a Linux file in Windows, you would get a mammoth line with no returns.)

Assuming that it is not an issue with the end of line characters in your text file, William Pursell's, Andy Lester's, or nullrevolution's answers will work.

A variation on the while read... suggestion:

while read -r line; do  echo "PREFIX " $line; done < $file

This could be run directly from the shell (no need for a batch / script file):

while read -r line; do echo "kp" $line; done < stusers.txt

A Perl way to do it would be:

perl -p -e's/^/prefix' filename

or

perl -p -e'$_ = "prefix $_"' filename

In either case, that reads from filename and prints the prefixed lines to STDOUT.

If you add a -i flag, then Perl will modify the file in place. You can also specify multiple filenames and Perl will magically do all of them.