find string in file using bash

Question

I need to find strings matching some regexp pattern and represent the search result as array for iterating through it with loop ), do I need to use sed ? In general I want to replace some strings but analyse them before replacing.

Answer 1

Using sed and diff:

sed -i.bak 's/this/that/' input
diff input input.bak

GNU sed will create a backup file before substitutions, and diff will show you those changes. However, if you are not using GNU sed:

mv input input.bak
sed 's/this/that/' input.bak > input
diff input input.bak

Another method using grep:

pattern="/X"
subst=that
while IFS='' read -r line; do
    if [[ $line = *"$pattern"* ]]; then
        echo "changing line: $line" 1>&2
        echo "${line//$pattern/$subst}"
    else
        echo "$line"
    fi  
done < input > output

Answer 2

The best way to do this would be to use grep to get the lines, and populate an array with the result using newline as the internal field separator:

#!/bin/bash

# get just the desired lines
results=$(grep "mypattern" mysourcefile.txt)

# change the internal field separator to be a newline
IFS=$'/n'

# populate an array from the result lines
lines=($results)

# return the third result
echo "${lines[2]}"

You could build a loop to iterate through the results of the array, but a more traditional and simple solution would just be to use bash's iteration:

for line in $lines; do
  echo "$line"
done