bash, adding string after a line - bash

Question

I'm trying to put together a bash script that will search a bunch of files and if it finds a particular string in a file, it will add a new line on the line after that string and then move on to the next file.
#! /bin/bash
echo "Creating variables"
SEARCHDIR=testfile
LINENUM=1
find $SEARCHDIR* -type f -name *.xml | while read i; do
echo "Checking $i"
ISBE=`cat $i | grep STRING_TO_SEARCH_FOR`
if [[ $ISBE =~ "STRING_TO_SEARCH_FOR" ]] ; then
echo "found $i"
cat $i | while read LINE; do
((LINENUM=LINENUM+1))
if [[ $LINE == "<STRING_TO_SEARCH_FOR>" ]] ; then
echo "editing $i"
awk -v "n=$LINENUM" -v "s=new line to insert" '(NR==n) { print s } 1' $i
fi
done
fi
LINENUM=1
done
the bit I'm having trouble with is
awk -v "n=$LINENUM" -v "s=new line to insert" '(NR==n) { print s } 1' $i
if I just use $i at the end, it will output the content to the screen, if I use $i > $i then it will just erase the file and if I use $i >> $i it will get stuck in a loop until the disk fills up.
any suggestions?

Answer 1

Unfortunately awk dosen't have an in-place replacement option, similar to sed's -i, so you can create a temp file and then remove it:
awk '{commands}' file > tmpfile && mv tmpfile file
or if you have GNU awk 4.1.0 or newer, the -i inplace is added, so you can do:
awk -i inplace '{commands}' file
to modify the original

Answer 2

#cat $i | while read LINE; do
# ((LINENUM=LINENUM+1))
# if [[ $LINE == "<STRING_TO_SEARCH_FOR>" ]] ; then
# echo "editing $i"
# awk -v "n=$LINENUM" -v "s=new line to insert" '(NR==n) { print s } 1' $i
# fi
# done
# replaced by
sed -i 's/STRING_TO_SEARCH_FOR/&\n/g' ${i}
or use awk in place of sed
also
# ISBE=`cat $i | grep STRING_TO_SEARCH_FOR`
# if [[ $ISBE =~ "STRING_TO_SEARCH_FOR" ]] ; then
#by
if [ $( grep -c 'STRING_TO_SEARCH_FOR' ${i} ) -gt 0 ]; then
# if file are huge, if not directly used sed on it, it will be faster (but no echo about finding the file)

Answer 3

If you can, maybe use a temporary file?
~$ awk ... $i > tmpfile
~$ mv tmpfile $i
Or simply awk ... $i > tmpfile && mv tmpfile $i
Note that, you can use mktemp to create this temporary file.
Otherwise, with sed you can insert a line right after a match:
~$ cat f
auie
nrst
abcd
efgh
1234
~$ sed '/abcd/{a\
new_line
}' f
auie
nrst
abcd
new_line
efgh
1234
The command search if the line matches /abcd/, if so, it will append (a\) the line new_line.
And since sed as the -i to replace inline, you can do:
if [[ $ISBE =~ "STRING_TO_SEARCH_FOR" ]] ; then
echo "found $i"
echo "editing $i"
sed -i "/STRING_TO_SEARCH_FOR/{a
\new line to insert
}" $i
fi