In my bash script, i am trying to execute following Linux command:
sed -i "/$data_line/ d" $data_dir
$data_line is entered by user and it may contain special characters that could brake regex. How can i escape all of the possible special characters in $data_line before i execute sed command?
You might be able to use this technique to protect the selector. The lines marked with "*****" below are the significant lines. The others are mostly for testing and demonstration. The key is to use a character that doesn't appear in the user input to delimit the selector address.
data_line='.*/ s/GOLD/LEAD/g;b;/.*' # scary user input
candidates='/:.|@#%^&;,!~abcABC' # ***** # (make it as long as you like)
char=$(echo "$candidates" | tr -d "$data_line") # *****
char=${char:0:1} # ***** choose the first candidate that doesn't appear in the user input
if [ -z "$char" ] # ***** this test checks for exhaustion of the candidate character set
then
echo "Unusable user input. Recommendation: cigarette and blindfold."
exit 1
fi
# test without protection
excitement="GOLD, I tell you, thar's GOLD in them thar hills!"
echo "$excitement" | sed "/$data_line/ d"
# output: "LEAD, I tell you, thar's LEAD in them thar hills!"
# test WITH protection
echo "$excitement" | sed "\\${char}${data_line}${char} d" # *****
# output: "GOLD, I tell you, thar's GOLD in them thar hills!"
# test WITH protection and useful user input
data_line="secret"
mystery="The secret map is tucked in a hidden compartment in my saddle bag."
echo -e "$excitement\n$mystery" | sed "\\${char}${data_line}${char} d"
# output: "GOLD, I tell you, thar's GOLD in them thar hills!"
char=${candidates//[$data_line]} instead of using echo and tr.grep -v -F "$data_line" "$data_dir" > ...
It's an old question, but for those still coming here, sed allows you to use a delimiter other than the forward slash.
For my script I switched from sed -i 's/$OLD_URL/$NEW_URL/g' to sed -i 's|$OLD_URL|$NEW_URL|g', as my URLs contained forward slashes, but not pipes.
# Source - https://stackoverflow.com/a/2933544
# Posted by Ignacio Vazquez-Abrams
# Retrieved 2025-11-07, License - CC BY-SA 2.5
grep -v -F "$data_line" "$data_dir" > ...
-v removes lines that match
-F uses string as is, special characters used without being special
