Rename all the files within a folder with prefix “Unix_”

Try this:

for f in *.txt; do d="$(head -1 "$f" | awk '{print $1}').txt"; if [ ! -f "$d" ]; then mv "$f" "$d"; else echo "File '$d' already exists! Skiped '$f'"; fi; done

or more long variant (as script):

#!/bin/sh
for f in *.txt; do
    d="$(head -1 "$f" | awk '{print $1}').txt"
    if [ ! -f "$d" ]; then
        mv "$f" "$d"
    else
        echo "File '$d' already exists! Skiped '$f'"
    fi
done

In case when destination file exists this one-liner skips it.

If you're using bash, then this one-liner can do it for you (assuming you have the variables $prefix, $suffix and $extension readily available)

mv "$filename" "$prefix${filename%.$extension}$suffix.$extension"

You could have your scripts be like this

#!/bin/bash
# Usage: addprefix <prefix> <files>

prefix=$1
shift
for f in "$@"
do
  mv "$f" "$prefix$f"
done

and

#!/bin/bash
# addsuffix <suffix> <files>

suffix=$1
shift
for f in "$@"
do
  extension=${f##*.}
  if [ -z $extension ]; then
    mv "$f" "$f$suffix"
  else
    mv "$f" "${f%.$extension}$suffix.$extension"
  fi
done