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Linux shell trap error

Linux shell trap error

The difference between a poor program and a good one is often measured in terms of the program's robustness. That is, the program's ability to handle situations in which something goes wrong.

As we recall from previous 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 status is anything other than zero, then the program failed in some way.

It is very important to check the exit status of programs we call in our scripts. It is also important that our scripts return a meaningful exit status when they finish. There was once a Unix system administrator who wrote a script for a production system containing the following 2 lines of code:.

Why is this such a bad way of doing it? It's not, if nothing goes wrong. That's the intended behavior. 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, the 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. There are several ways we can get and respond to the exit status of a program.

We can see this work with the following:. The true and false commands are programs that do nothing except return an exit status of zero and one, respectively.

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 script with an exit status of 1. While this is a working solution to the problem, there are more clever methods that will save us some typing.

The next approach we can try is to use the if statement directly, since it evaluates the exit status of commands it is given. Here we check to see if the cd command is successful. Only then does rm get executed; otherwise an error message is output and the program exits with a code of 1, indicating that an error has occurred. This is a safety precaution. The reason is a little subtle and has to do with the lax way Unix-like systems name files.

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Since it is possible to include almost any character in a file name, we must card against file names that begin with hyphens as thy might be interpreted as command options after the wildcard is expanded. For example, if there was a file named -rf in the directory, it might cause rm to do unpleasant things. It's a good idea to always include ". Since we will be checking for errors often in our programs, it makes sense to write a function that will display error messages.

This will save more typing and promote laziness. To explain how they work, here is a quote from the bash man page:.Join Stack Overflow to learn, share knowledge, and build your career. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information.

However, when using set -e it is not trapped. Without set -e ERR is trapped. If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment.

The ERR trap is normally not inherited in such cases. Without -e : The ls command fails inside your function, and, due to being the last command in the function, the function reports ls 's nonzero exit code to the caller, your top-level script scope.

In that scopethe ERR trap is in effect, and it is invoked but note that execution will continue, unless you explicitly call exit from the trap. With -e but without -E : The ls command fails inside your functionand because set -e is in effect, Bash instantly exits, directly from the function scope - and since there is no ERR trap in effect there because it wasn't inherited from the parent scopeyour trap is not called.

While the man page is not incorrect, I agree that this behavior is not exactly obvious - you have to infer it. Learn more. Asked 4 years, 10 months ago.

linux shell trap error

Active 6 months ago. Viewed 19k times. I have a simple script :! According to the bash man page, for set -e A trap on ERR, if set, is executed before the shell exits. Why isn't my trap executed? From the man page it seems like it should. As an aside: it's better to single-quote your trap handlers, unless you explicitly want variable references in it expanded up front.

Active Oldest Votes. In short: use set -eE in lieu of just set -e :! What I believe is happening: Without -e : The ls command fails inside your function, and, due to being the last command in the function, the function reports ls 's nonzero exit code to the caller, your top-level script scope. Thanks, I will.

You need to use set -o errtrace for the function to inherit the trap. Not necessarily, if you use shopt -s extdebug.

Shell Scripting Tutorial for Beginners 27 - Signals and Traps

Ritesh Ritesh 1, 11 11 silver badges 15 15 bronze badges. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Episode Gaming PCs to heat your home, oceans to cool your data centers.Hey you Word on the street is your shell scripts don't do any error handling.

They just chug happily along even when everything is broken. In this mode any command your script runs which returns a non-zero exitcode - an error in the world of shell - will cause your script to itself terminate immediately with an error. Yes, this is what you want. A neat predictable failure is infinitely better than a noisy unreliable failure.

Oh and as long as you're messing with shell modes, -e goes well with -x which I like to think of as shell X-ray. A trap is a snippet of code that the shell executes when it exits or receives a signal. I find traps most useful for making sure my scripts clean-up after themselves whatever happens e. Note that you can only set one trap per signal. If you set a new trap you're implicitly disabling the old one. You can also disable a trap by specifying - as the argument, like this:.

Not propagating signals in this manner is being a bad Unix citizen. Or is there a reason you should kill the PID instead? Theorically, if you resend a signal it will be trap again and loop. Even if it works, it is not as clean. You also need to disable the corresponding trap, before raising the signal again.

Two simple tricks for better shell script error handling

So the code should be something like. I think most people are guily of this. I know I have been but I am forcing myself to do it now. Programming in anything else is, at best, unreliably portable. For instance, tar exits with 1 if a file changes during an archive create -cand it'll exit with 2 if a more serious error occurs.

So, if you use the -e approach, you'll have to do:. I have wrote the below script but it asking password for each server.

Sc,Calcutta 1,M. Sc,Stanford 2,M. Tech,Shibpur 4,M.There is a simple, useful idiom to make your bash scripts more robust - ensuring they always perform necessary cleanup operations, even when something unexpected goes wrong. The secret sauce is a pseudo-signal provided by bash, called EXIT, that you can trap ; commands or functions trapped on it will execute when the script exits for any reason.

Let's see how this works.

linux shell trap error

You place any code that you want to be certain to run in this "finish" function. A good common example: creating a temporary scratch directory, then deleting it after. Another scenario: Imagine you are automating some system administration task, requiring you to temporarily stop a server Then the pattern is:. A concrete example: suppose you have MongoDB running on an Ubuntu server, and want a cronned script to temporarily stop the process for some regular maintenance task.

The way to handle it is:. There is another situation where the exit trap is very useful: if your script initiates an expensive resource, needed only while the script is executing, and you want to make certain it releases that resource once it's done.

If you're not familar with this: Servers running on the Amazon cloud are called " instances ". Instances are launched from Amazon Machine Images, a. AMIs are kind of like a snapshot of a server at a specific moment in time. That last step is really important.

If your script fails to terminate the instance, it will keep running and accruing charges to your account. In the worst case, you won't notice until the end of the month, when your bill is way higher than you expect.

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Believe me, that's no fun! If our AMI-creation is encapsulated in a script, we can set an exit trap to destroy the instance. Let's rely on the EC2 command line tools:. At this point in the script, the instance EC2 server is running [2]. You can do whatever you like: install software on the instance, modify its configuration programatically, et cetera, finally creating an image from the final version. The instance will be terminated for you when the script exits - even if some uncaught error causes it to exit early.

Just make sure to block until the image creation process finishes. I believe what I've covered in this article only scratches the surface; having used this bash pattern for years, I still find new interesting and fun ways to apply it. You will probably discover your own situations where it will help make your bash scripts more reliable. But using the scratch file makes the code a bit more readable, leaves us with better logging for debugging, and makes it easy to capture other info from ec2-run-instances's output if we wish.

The basic code structure is like this:!If you've written any amount of bash code, you've likely come across the trap command.

Trap allows you to catch signals and execute code when they occur. Signals are asynchronous notifications that are sent to your script when certain events occur. Most of these notifications are for events that you hope never happen, such as an invalid memory access or a bad system call.

linux shell trap error

There are also "user" events available that are never generated by the system that you can generate to signal your script. The man page for signal 7 describes all the available signals. The Wikipedia page for signal IPC has a bit more detail. As I mentioned, most of them are of little interest in scripts. If you don't want your script to be stopped like this, you can trap the signal and remind yourself that interrupting the script is to be avoided.

The most common use of the trap command though is to trap the bash-generated psuedo-signal named EXIT. Say, for example, that you have a script that creates a temporary file. Rather than deleting it at each place where you exit your script, you just put a trap command at the start of your script that deletes the file on exit:. Now whenever your script exits, it deletes your temporary file.

If your cleanup needs are complex, you don't have to try to jam it all into a string with semicolons, just write a function:. Note that if you send a kill -9 to your script, it will not execute the EXIT trap before exiting. The other possible thing that you might like to use the trap command for is to catch Ctrl-C so that your script can't be interrupted or perhaps so you can ask if the user really wants to interrupt the process.

As an example, I'll use the following handler function, which warns the user on the first two Ctrl-Cs and then exits on the third:.

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My first shot at the test script had the sleep 10 as the while condition:. But that didn't work. After a bit of thought, I realized it's because when the trap command returns, it does not resume the "sleep" command at the point it was interrupted, nor does it restart the "sleep" command, rather it returns to the next command after the command that was interrupted, which in this case is whatever follows the while loop.

So the loop ends and the script exits normally. This is an important point: interrupted commands are not restarted. So if your script needs to do something important that shouldn't be interrupted, then you can't, for example, use the trap command to trap the signal, print a warning, and then resume the operation like nothing happened.

Rather, what you need to do if you can't have something interrupted is disable Ctrl-C handling while the command executes. You can do this with the trap command too by specifying an empty command to trap. You can also use the trap command to reset signal handling to the default by specifying a "-" as the command. So you might do something like this:.

So unless your script has long moments when it's just waiting, trapping signals and actually doing something may not provide the experience you hoped for. The final thing I want to look at is trapping user-defined signals to a script. Say that I want to monitor the system log and count the number of times that sudo is run, and I want to run the script in the background and then send it a signal whenever I want it to display the count:.

What this does is pipes the output from journalctl i.

Errors and Signals and Traps (Oh My!) - Part 1

Inside the loop, the if-statement checks to see if the line is a sudo command. If so, it increments a counter. Unfortunately, once again, this failed to work. The first problem I discovered, which I recently mentioned in my post on Job Control is that if the sudo command needs to prompt for a password, the script becomes suspended just after starting.

If you need to test something with sudo and want to make sure that everything is working both when sudo prompts for a password and when it does not prompt for a password, execute the command sudo -k to reset sudo's password timestamp.

After executing sudo with the -k option, sudo will once again ask for a password the next time it's run, regardless of how recently you entered a password. After figuring out the suspended background problem, I figured all systems were "go", but not so, still nothing.

The problem now is because the loop is taking input from a pipe. The original bash process has now executed one sub-process for "journalctl" and another sub-process for "while read lineFinally, there is a definitive guide for bash error handling.

I have outlined the foundation needed to handle any error in bash. Knowing how to use exit codes, options such as errexit, and traps allow you to create robust scripts and better handle errors in bash.

Handling errors based on exit codes is the standstill method for detecting failure in a command. This is especially true in the case of external commands. Curl in bash is a good example of how to handle errors based on known error codes. Unlike user-defined functions, you can expect external command errors codes to be well documented. An exit code of 0 means the command executed without any issues.

Otherwise, something went wrong. In fact, you could get away with bash error handling using only exit codes. You could try until you find a lazier solution. At least that is what any person would do after writing a few conditionals to handle errors based on error codes. What if you just want your bash script to die in the case that something goes wrong to minimize the damage caused by a script with errors?

But would it be nice to stop after hitting the first mine? There is a way to do that in bash, i. For example, what if we want to handle the error of something instead of falling back to exit? Within a bash script, you may use the set command to turn exit on error on, set -e. For more details on errexit, see how to debug bash script. Trap allows us to set commands to run in the event the shell receives a signal.

More signals may be listed using the trap -l command. For example, we may want to check what commands are associated with the ERR signal. In that case, we could type a command line as follows. Although the above example may seem trivial at first, it is exactly where you would start to build a robust error handling function like error.

It is important to note that exit on error will not occur if the end of fallback does not return a nonzero exit code.

Linux trap and onitr command

The above example would be useful if enabling potential recovery in the fallback. Conversely, if the intent of the fallback is to act as a trap on an instance of the ERR signal, then adjustments would be required as follows.

Now we have everything we need to do error handling in bash. Here follow some advice I have to handle errors in bash. Not reusing code and inventing things you already know how to do, now that is hard. Here are some pointers that I have to make handling errors easier.

Most of what is seen above for doing it the hard way can be put in your boilerplate code. Exit on error functions to be called in traps, the works. Code consistency in error handling reduces the chance of unintendedly failing to handle an error. This is especially important when coding with errexit set on. Error handling comes into handy when you know something will succeed eventually but requires failure along the way.

Have a fallback and only allow it to be used once. For code with a known exception, handle errors if recovery is possible, otherwise fallback. As you may have guessed our fallback is to exit with a message telling users where and what happened if any. Record errors in a log file or database, whatever method you prefer. After all, unless you are debugging bash scriptsyou are not going to be around when a bird flies into a ceiling fan.Share to TwitterShare to FacebookShare to Pinterest Labels: Free Tips Newer Post Older Post Home Subscribe to: Post Comments (Atom) Translate Subscribe to our Newsfeed Top 10 Post Best Football Predictions Tips For 19th September 2017 Click To Learn How To Pay For Your Premium Subscription Below are the free predictions for 19th September, 2017.

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