Shell Script to Replace or Insert Lines of Text in File - shell

Question

I wrote a Shell script to replace text in the Jenkinsfiles of a large number of repos. I had used this to add on a parameter to a single line of text. However, now I need to insert a line of text before and after an existing command in the Jenkinsfiles. I don't have too much shell experience and could use some help.
Here is the Jenkinsfile text before:
sh "chmod +x increment.sh"
def result = sh returnstdout:true, script: "./increment.sh '${Version}' '${ReleaseVersion}' '${GitRepoURL}' '${CutRelease}' '${Branch}' '${JiraID}'"
//echo "$result"
I need to add the following before the "def result" line:
sshagent(['gitssh']) {
and then add a closing curly bracket after the "def result" line:
}
I need the end result to be:
sh "chmod +x increment.sh"
sshagent(['gitssh']) {
def result = sh returnstdout:true, script: "./increment.sh '${Version}' '${ReleaseVersion}' '${GitRepoURL}' '${CutRelease}' '${Branch}' '${JiraID}'"
}
//echo "$result"
I actually don't care about keeping the commented out echo command if that makes it more difficult, but it is just to show what I have surrounding the "def result" line.
How can I accomplish this end result?
If it helps, I previously was adding new parameters at the end of the "def result" line with this code:
if [ -e Jenkinsfile ]
then
sed -i -e "s/\${Branch}/\${Branch}\' \'\${JiraID}/g" Jenkinsfile
fi
Note: I am on a Mac.
Code so far:
file=repos_remaining.txt
while IFS="," read -r repoURL repoName; do
echo $repoURL
cd ..
echo $repoName
cd $(echo $repoName | tr -d '\r')
file=repos_remaining.txt
if [ -e Jenkinsfile ]
then
# sed -i -e $"s/def result/sshagent([\'gitssh\']) {\
# def result/g" Jenkinsfile
fi
# git add "Jenkinsfile"
# git commit -m "Added JiraID parameter to Jenkinsfile"
# git push origin master
done < "$file"

Answer 1

As with most cases where people want to automate the editing of a file, I suggest using ed:
ed -s Jenkinsfile <<'EOF'
/^def result/i
sshagent(['gitssh']) {
.
.+1a
}
.
w
EOF
The commands in the heredoc tell ed to move the cursor to the first line starting with def result, insert a line above it, append a line after it, and finally write the modified file back to disc.