Sed Command:How to Replace If Exists Else Just Insert

Replace only if string exists in current line

Solution

Assuming your input file $target contains the following:

some text mystring some other text
some text mystring a searchstring
just some more text

This command:

sed -i -e '/searchstring/ s/mystring/1/ ; /searchstring/! s/mystring/0/' $target

will change its content to:

some text 0 some other text
some text 1 a searchstring
just some more text

Explanation

The script contains two substitute (s) commands separated by a semicolon.

The substitute command accepts an optional address range that select which lines the substitution should take place.

In this case regexp address was used to select lines containing the searchstring for the first command; and the lines that do not contain the searchstring (note the exclamation mark after the regexp negating the match) for the second one.

Edit

This command will perform better and produce just the same result:

sed -i -e '/searchstring/ s/mystring/1/ ; s/mystring/0/' $target

The point is that commands are executed sequentially and thus if there is still a mystring substring in the current line after the first command finished then there is no searchstring in it for sure.

Kudos to user946850.

Replace whole line containing a string using Sed

You can use the change command to replace the entire line, and the -i flag to make the changes in-place. For example, using GNU sed:

sed -i '/TEXT_TO_BE_REPLACED/c\This line is removed by the admin.' /tmp/foo

Bash creating a string, if string already exists replace it

if there is the few text, it should replace it, if few text is missing, just create it on the first line.

That's an if.

if <file has text>; then <replace text>; else <add text to first line>; fi

or in bash:

file=/usr/local/sbin/.myappenv
if grep -q few "$file"; then
     sed 's/few/asd/' "$file"
else
     { 
        echo asd
        cat "$file"
     } > "$file".tmp
     mv "$file".tmp "$file"
fi

How to test if string exists in file with Bash? https://unix.stackexchange.com/questions/99350/how-to-insert-text-before-the-first-line-of-a-file and https://mywiki.wooledge.org/BashGuide/TestsAndConditionals . You might interest yourself in some automation methods, like ansible lineinfile or chezmoi depending on the goal.

How to swap text based on patterns at once with sed?

Maybe something like this:

sed 's/ab/~~/g; s/bc/ab/g; s/~~/bc/g'

Replace ~ with a character that you know won't be in the string.

sed edit file in place

The -i option streams the edited content into a new file and then renames it behind the scenes, anyway.

Example:

sed -i 's/STRING_TO_REPLACE/STRING_TO_REPLACE_IT/g' filename

while on macOS you need:

sed -i '' 's/STRING_TO_REPLACE/STRING_TO_REPLACE_IT/g' filename

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