Error In Unix Shell Script
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and Signals and Traps (Oh My!) - Part 1 by William Shotts, Jr. In this lesson, we're going to look at handling errors during the execution of your scripts. The difference between a good program and a poor one is often measured in terms of the program's robustness. That is, the error handling in shell script unix program's ability to handle situations in which something goes wrong. Exit status As you recall from previous
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lessons, every well-written program returns an exit status when it finishes. If a program finishes successfully, the exit status will be zero. If the exit
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status is anything other than zero, then the program failed in some way. It is very important to check the exit status of programs you call in your scripts. It is also important that your scripts return a meaningful exit status
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when they finish. I once had a Unix system administrator who wrote a script for a production system containing the following 2 lines of code: # Example of a really bad idea cd $some_directory rm * Why is this such a bad way of doing it? It's not, if nothing goes wrong. The two lines change the working directory to the name contained in $some_directory and delete the files in that directory. That's the intended behavior. But what happens if the directory named unix shell scripting examples programs in $some_directory doesn't exist? In that case, the cd command will fail and the script executes the rm command on the current working directory. Not the intended behavior! By the way, my hapless system administrator's script suffered this very failure and it destroyed a large portion of an important production system. Don't let this happen to you! The problem with the script was that it did not check the exit status of the cd command before proceeding with the rm command. Checking the exit status There are several ways you can get and respond to the exit status of a program. First, you can examine the contents of the $? environment variable. $? will contain the exit status of the last command executed. You can see this work with the following: [me] $ true; echo $? 0 [me] $ false; echo $? 1 The true and false commands are programs that do nothing except return an exit status of zero and one, respectively. Using them, we can see how the $? environment variable contains the exit status of the previous program. So to check the exit status, we could write the script this way: # Check the exit status cd $some_directory if [ "$?" = "0" ]; then rm * else echo "Cannot change directory!" 1>&2 exit 1 fi In this version, we examine the exit status of the cd command and if it's not zero, we print an error message on standard error and terminate the s
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the unix shell scripting for loop company Business Learn more about hiring developers or posting ads with us Unix & Linux unix shell scripting w3schools Questions Tags Users Badges Unanswered Ask Question _ Unix & Linux Stack Exchange is a question and answer site for users of unix shell scripting if else Linux, FreeBSD and other Un*x-like operating systems. Join them; it only takes a minute: Sign up Here's how it works: Anybody can ask a question Anybody can answer The best answers are voted up and rise to the http://linuxcommand.org/wss0150.php top Bash if statement [: missing `]' error [duplicate] up vote 3 down vote favorite 2 This question already has an answer here: Brackets in if condition: why am I getting syntax errors without whitespace? 3 answers I am having trouble with bash. I am trying to put a command in an if statement and then compare it to a string. This works perfectly. echo $(ipcs | grep Shared | awk '{print $2}') When I http://unix.stackexchange.com/questions/160180/bash-if-statement-missing-error put it in an if statement I get some problems. $ if [ "$(ipcs | grep Shared | awk '{print $2}')" -eq "Shared"]; then echo expression evaluated as true; else echo expression evaluated as false; fi bash: [: missing `]' expression evaluated as false $ if [ "$(ipcs | grep Shared | awk '{print $2}')" = "Shared"]; then echo expression evaluated as true; else echo expression evaluated as false; fi bash: [: missing `]' expression evaluated as false $ if [ "$(ipcs | grep Shared | awk '{print $2}')" == "Shared"]; then echo expression evaluated as true; else echo expression evaluated as false; fi bash: [: missing `]' expression evaluated as false I tried ==, =, and -eq because I wasn't sure which one to use. bash shell-script test share|improve this question edited Oct 9 '14 at 21:50 Gilles 371k686741125 asked Oct 9 '14 at 10:28 cokedude 89139 marked as duplicate by Gillesbash Users with the bash badge can single-handedly close bash questions as duplicates and reopen them as needed. Oct 9 '14 at 21:50 This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question. For those wondering: these are not multi-line statements! –Anthon Oct 9 '14 at 10:45 If you just want to
article shows how to add error checking to highlight problems you might otherwise miss. Access the shell on Mac via the Terminal.app Unlike more recently designed languages, shell script does not have an easy answer for error handling. https://www.dssw.co.uk/blog/2012-10-08-how-to-add-error-checking-to-your-shell-scripts/ There are no common exception handling routines or ways of wrapping up large blocks of script and asking for errors to fall through to a provided subroutine. Instead shell script asks you, the author, to check individual program exit codes http://stackoverflow.com/questions/12335356/how-to-handle-error-exception-in-shell-script and branch as needed in case of an error. In practice, this means your once short script is going to get a little longer and a little more involved. Unix Exit Status Each program you run through a shell script returns shell script an exit status. This numeric status value tells the calling script if the program completed successfully or if an error was encountered. The exit status is not visible on the command line. This makes it difficult to tell if something went wrong just by looking at the textual output of a shell script. It is possible - even common - for scripts to print nothing and yet encounter multiple errors. Let's consider this simple script: #!/bin/sh cp /Volumes/Documents/criticalfile.txt /Volumes/BackUp/. This script unix shell script does one thing; it copies a single file from one volume to another using the cp program. Thankfully the cp program is chatty and will print a message if an error is encountered. That is great for us reviewing the output visually, but for the shell running our script the error will go completely unnoticed. This is a problem if our script goes on to do more work, or if we want the script to robustly deal with errors. Let's add error checking to this simple script. #!/bin/sh cp /Volumes/Documents/criticalfile.txt /Volumes/BackUp/. if [ "$?" != "0" ]; then echo "[Error] copy failed!" 1>&2 exit 1 fi We have added an if/fi block below the cp line. The new block checks the special variable $? to see if it equals 0 or not. Unix programs should return 0 if they completed successfully. Any other value means something went wrong. The exact meaning of the returned value is frequently documented in the program's man page. If an error is detected in our script's if/fi block, then a message is printed and the script immediately exits also reporting an error. Why report another error? If our script does not explicitly say exit 1 then the script is assumed to have completed successfully. The error from cp does not matter unless we explicitly make it matter by passing it to our script's caller. William Shotts, Jr's article Errors and Si
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up How to handle error/exception in shell script? up vote 6 down vote favorite 1 Below is my script that I am executing in the bash. And it works fine. fileexist=0 for i in $( ls /data/read-only/clv/daily/Finished-HADOOP_EXPORT_&processDate#.done); do mv /data/read-only/clv/daily/Finished-HADOOP_EXPORT_&processDate#.done /data/read-only/clv/daily/archieve-wip/ fileexist=1 done Problem Statement:- In my above shell script which has to be run daily using cron job, I don't have any error/exception handling mechanism. Suppose if anything gets wrong then I don't know what's has happened? As after the above script is executed, there are some other scripts that will be dependent on the data provided by above script, so I always get's complaint from the other people who are depending on my script data that something wrong has happened. So is there any way I can get notified if anything wrong has happened in my script? Suppose if the cluster is having some maintenance and at that time I am running my script, so definitely it will be failing for sure, so can I be notified if my above scripts failed, so that I will be sure something wrong has happened. Hope my question is clear enough. Any thoughts will be appreciated. linux bash shell unix share|improve this question edited Sep 10 '12 at 2:52 asked Sep 8 '12 at 23:38 lining 6,10044162271 @Levon, Done. Now it's better? –lining Sep 8 '12 at 23:48 Yes, I think this looks much better and more readable, won't intimidate potential helpers away :) –Levon Sep 8 '12 at 23:49 Yup.. Thanks for the advice. I will