How do I get the output of a shell command executed using into a variable from Jenkinsfile (groovy)?

The latest version of the pipeline sh step allows you to do the following;

// Git committer email
GIT_COMMIT_EMAIL = sh (
    script: 'git --no-pager show -s --format=\'%ae\'',
    returnStdout: true
).trim()
echo "Git committer email: ${GIT_COMMIT_EMAIL}"

Another feature is the returnStatus option.

// Test commit message for flags
BUILD_FULL = sh (
    script: "git log -1 --pretty=%B | grep '\\[jenkins-full]'",
    returnStatus: true
) == 0
echo "Build full flag: ${BUILD_FULL}"

These options where added based on this issue.

See official documentation for the sh command.

For declarative pipelines (see comments), you need to wrap code into script step:

script {
   GIT_COMMIT_EMAIL = sh (
        script: 'git --no-pager show -s --format=\'%ae\'',
        returnStdout: true
    ).trim()
    echo "Git committer email: ${GIT_COMMIT_EMAIL}"
}

Current Pipeline version natively supports returnStdout and returnStatus, which make it possible to get output or status from sh/bat steps.

An example:

def ret = sh(script: 'uname', returnStdout: true)
println ret

An official documentation.

quick answer is this:

sh "ls -l > commandResult"
result = readFile('commandResult').trim()

I think there exist a feature request to be able to get the result of sh step, but as far as I know, currently there is no other option.

EDIT: JENKINS-26133

EDIT2: Not quite sure since what version, but sh/bat steps now can return the std output, simply:

def output = sh returnStdout: true, script: 'ls -l'