Bash while loop search and replace using sed

Question

I have a problem with my following script (this is the relevant part of it):

#!/bin/bash OLD=( "_MAIN1_" "_MAIN2_" ) NEW=( "#111" "#222" ) length=${#OLD[*]} i=0 while (( i < length )) do sed -e "s/${OLD[$i]}/${NEW[$i]}/g" oldfile.txt > newfile.txt #sed -e 's/_MAIN1_/#111/g' oldfile.txt > newfile.txt # this works # Another way that does not work #sed -e 's/'"${OLD[$i]}"'/'"${NEW[$i]}"'/g' oldfile.txt > newfile.txt ((i++)) done exit 0 

My goal is to replace strings in a file and save it into a new one. The "old" and "new" strings are stored in an array.

I tried a lot of things and played around with single and double quotes - but nothing worked. When I echo the variables I get the correct strings inside the loop. If explicit two strings are set in sed command it works fine for this.

The string patterns follow those in my example arrays ('new' contains the underscore "_" and 'old' contains the hashtag "#").

I'm running bash on a Ubuntu 16.04 box.

Thank you very much!

Answer 1

Create a sed script that does all the substitutions, and then apply that sed script to your file.

for (( i=0; i<${#OLD[@]}; ++i )); do printf 's/%s/%s/g\n' "${OLD[$i]}" "${NEW[$i]}" done >script.sed sed -f script.sed inputfile >outputfile && mv outptufile inputfile && rm script.sed 

This way you limit the number of times that you need to parse the input file to one.

For the given data in OLD and NEW the sed script will be generated as

s/_MAIN1_/#111/g s/_MAIN2_/#222/g 

Answer 2

Your entire script (which doesn't work) can be replaced with the single line:

sed 's/_MAIN\([12]\)_/#\1\1\1/g' oldfile.txt > newfile.txt 

Or, more readably but equivalently:

sed 's/_MAIN1_/#111/g;s/_MAIN2_/#222/g' oldfile.txt > newfile.txt 

There are several dozen other ways to accomplish this, depending on your actual use case (e.g., where the data comes from, what the values really look like, what you are trying to do with the data, etc.).

Answer 3

Thanks to Rakesh Sharma's comment I could solve this by using the -i flag in sed command. Before the loop the original file is backuped:

cp oldfile.txt oldfile.backup while (( i < length )) do sed -i -e 's/'"${OLD[$i]}"'/'"${NEW[$i]}"'/g' oldfile.txt ((i++)) done 

The next time I will use Perl for string manipulations.

Edit: Added ((i++)) to prevent an infinite loop (Thanks to Kusalananda's comment).