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#!/bin/bash
# subst.sh: a script that substitutes one pattern for
#+ another in a file,
#+ i.e., "sh subst.sh Smith Jones letter.txt".
# Jones replaces Smith.
ARGS=3 # Script requires 3 arguments.
E_BADARGS=85 # Wrong number of arguments passed to script.
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` old-pattern new-pattern filename"
exit $E_BADARGS
fi
old_pattern=$1
new_pattern=$2
if [ -f "$3" ]
then
file_name=$3
else
echo "File \"$3\" does not exist."
exit $E_BADARGS
fi
# -----------------------------------------------
# Here is where the heavy work gets done.
sed -e "s/$old_pattern/$new_pattern/g" $file_name
# -----------------------------------------------
# 's' is, of course, the substitute command in sed,
#+ and /pattern/ invokes address matching.
# The 'g,' or global flag causes substitution for EVERY
#+ occurence of $old_pattern on each line, not just the first.
# Read the 'sed' docs for an in-depth explanation.
exit $? # Redirect the output of this script to write to a file.
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