仅当变量具有值时sed bash替换

Question

I'm trying to find a way using variables and sed to do a specific text substitution using a changing input file, but only if there is a value given to replace the existing string with. No value= do nothing (rather than remove the existing string).

Example Substitute.csv contains 5 lines=

this-has-text
this-has-text

this-has-text
this-has-text

and file.text has one sentence=

"When trying this I want to be sure that text-this-has is left alone."

If I run the following command in a shell script

Text='text-this-has'
Change=`sed -n '3p' substitute.csv`
grep -rl $Text /home/username/file.txt | xargs sed -i "s|$Text|$Change|"

I end up with

"When trying this I want to be sure that is left alone."

But I'd like it to remain as

"When trying this I want to be sure that text-this-has is left alone."

Any way to tell sed "If I give you nothing new, do nothing"?

---

I apologize for the overthinking, bad habit. Essentially what I'd like to accomplish is if line 3 of the csv file has a value - replace $Text with $Change inline. If the line is empty, leave $Text as $Text.

Answer 1

Text='text-this-has'
Change=$(sed -n '3p' substitute.csv)
if [[ -n $Change ]]; then
    grep -rl $Text /home/username/file.txt | xargs sed -i "s|$Text|$Change|"
fi

Answer 2

Just keep it simple and use awk:

awk -v t="$Text" -v c="$Change" 'c!=""{sub(t,c)} {print}' file

If you need inplace editing just use GNU awk with -i inplace .

Given your clarified requirement, this is probably what you actually want:

awk -v t="$Text" 'NR==FNR{if (NR==3) c=$0; next} c!=""{sub(t,c)} {print}' Substitute.csv file.txt

Answer 3

Testing whether $Change has a value before launching into the grep and sed is undoubtedly the most efficient bash solution, although I'm a bit skeptical about the duplication of grep and sed ; it saves a temporary file in the case of files which don't contain the target string, but at the cost of an extra scan up to the match in the case of files which do contain it.

If you're looking for typing efficiency, though, the following might be interesting:

find . -name '*.txt' -exec sed -i "s|$Text|${Change:-&}|" {} \;

Which will recursively find all files whose names end with the extension .txt and execute the sed command on each one. ${Change:-&} means "the value of $Change if it exists and is non-empty, and otherwise an & "; & in the replacement of a sed s command means "the matched text", so s|foo|&| replaces every occurrence of foo with itself. That's an expensive no-op but if your time matters more than your cpu time, it might have been worth it.