By Ron Peters
Procedure directors desire libraries of options which are creative yet comprehensible. They don't are looking to reinvent the wheel, yet they don't are looking to reinvent filesystem administration both! specialist Shell Scripting is the final word source for all operating Linux, Unix, and OS X procedure directors who want to have brief, succinct, and robust shell implementations of difficult procedure scripting tasks.
* Automating small to medium procedure administration tasks
* studying method info and enhancing configuration files
* Scripting Linux, Unix, and OS X functions utilizing bash, ksh, et al.
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Extra info for Expert Shell Scripting
Otherwise you have to add 13 to the month and subtract 1 from the year. The following code calculates the day value for today. Once you know this, you subtract the epoch value from that value to get the number of days since the start of the epoch. The script then outputs that value. The output is left unformatted in case you want to use the number as input for another command or process. today_days=$((($year*365)+($year/4)-\ ($year/100)+($year/400)+\ ($month*306001/10000)+$day)) days_since_epoch=$(($today_days-$epoch_days)) echo $days_since_epoch You may find it useful to have two versions of this script: one that outputs the elapsed time in days and the other that outputs it in seconds.
Thus, both expressions determine if a variable has been assigned a value. The following expressions are similar to the previous ones; they determine if a variable is undefined: if [ -z $string ];then... if [ ! $any_variable ];then... These conditional statements are simply the negations of the corresponding expressions in the previous examples (-z tests whether a string has zero length, and ! is the logical NOT symbol). The next example uses a couple of features, one of which we have already seen.
Once the last digit is found, the function returns a true value of 1 if the digit is an even number and a false of 0 if it is not. I will discuss this command and others like it in Chapter 24. \)$/\2/'` case $LAST_DIGIT in 0|2|4|6|8 ) return 1 ;; *) return 0 ;; esac } This second function is one I have used many times. It determines whether a remote system is running and connected to the network. It works by pinging the specified machine three times and sending the output to /dev/null. Success is based on the value of $?