I have my.cnf file:

[mysqld]
log-bin = mysql-bin
relay-log = relay-bin
#skip-slave-start
......

If there is no "skip-slave-start", I want to add it.
Or uncomment if it presents and commented.
Could someone tell me how to implement this feature in the easiest way?

try this shell script:

#!/bin/bash
grep '^skip-slave-start' my.cnf >& /dev/null
if [ $? -ne 0 ] ; then
    echo "skip-slave-start is not turned on, so going to turn it on..."
    grep '^ *# *skip-slave-start' my.cnf >& /dev/null
    if [ $? -eq 0 ] ; then
    # skip-slave-start is commented
        sed -i 's/^ *# *skip-slave-start/skip-slave-start/' my.cnf
            # so uncommnt it
    else
    # skip-slave-start is not present
        sed -i 's/\[mysqld\]/[mysqld]\nskip-level-start/' my.cnf
            # so add it in [mysqld] section
    fi
else
    echo "skip-slave-start was turned on"
fi