Capture Error Messages Unix
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How To Suppress Error Messages In Unix
Posts Go to Page... learn unix and linux commands How to capture actual error linux capture error output message when a command fails to execute Shell Programming and Scripting Thread Tools Search this Thread Display solaris error messages Modes #1 04-26-2009 prathima Registered User Join Date: Apr 2008 Last Activity: 26 April 2009, 12:49 PM EDT Posts: 5 Thanks: 0 Thanked 0 Times in 0 Posts How to capture actual error
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message when a command fails to execute I want to capture actual error message in case the commands I use in my shell script fails. For eg: ls -l abc.txt 2>>errorlog.txt In this case I understand the error message is written to the errorlog.txt and I assume its bacause the return code from the command ls -l abc might return 2 if "abc" doesnt exists. My question is: How about if the command return
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non zero return code and is not equal to 2? I want to do something like this: ls -l abc.txt if [$? -ne 0] then echo $errorMessage >> errorlog.txt fi where I want actual error message that the command has returned to be written to errorlog.txt. Just for understanding I used $errorMessage but I assume there should be some means to capture the actual error message which can later be stored in errorMessage or written directly to errorlog.txt. Can someone please help? Remove advertisements Sponsored Links prathima View Public Profile Find all posts by prathima #2 04-26-2009 devtakh Unix Enthusiatic Join Date: Oct 2007 Last Activity: 21 August 2013, 5:20 AM EDT Location: Bangalore Posts: 738 Thanks: 0 Thanked 7 Times in 7 Posts Quote: Originally Posted by prathima I want to capture actual error message in case the commands I use in my shell script fails. For eg: ls -l abc.txt 2>>errorlog.txt In this case I understand the error message is written to the errorlog.txt and I assume its bacause the return code from the command ls -l abc might return 2 if "abc" doesnt exists. Quote: Here, 2 is not the return code. 2 is the File descriptor value(FD). By default, the standard inputs FD is 0, std o/p is 1 and std error is
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 windows error messages program's ability to handle situations in which something goes wrong. Exit status As you recall from
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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 oracle error messages 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 you call in your scripts. It is also important that your scripts return a meaningful exit http://www.unix.com/shell-programming-and-scripting/108220-how-capture-actual-error-message-when-command-fails-execute.html status 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 http://linuxcommand.org/wss0150.php named 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 termi
Mon, 2010/04/05 - 10:30 - 31 comments | Latest by Liraz Siri Psssst. Hey you... yeah you. Word on the street is your shell scripts don't do any error handling. They just chug happily along even https://www.turnkeylinux.org/blog/shell-error-handling when everything is broken. Because a lowly shell shell script doesn't need any error handling right? WRONG! Here are two simple tricks that are easy to use and will make your scripts much more robust. Turn http://knowledgebase.progress.com/articles/Article/26971 on -e mode (do you feel lucky - punk?) 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 error messages terminate immediately with an error. You can do that in your shebang line: #!/bin/sh -e Or using set: set -e Yes, this is what you want. A neat predictable failure is infinitely better than a noisy unreliable failure. If you REALLY want to ignore an error, be explicit about it: # I don't care if evil-broken-command fails evil-broken-command || true Oh and as long as you're messing with shell modes, -e goes well with capture error messages -x (which I like to think of as shell X-ray). Like this: #!/bin/sh -ex Or like this: # turn -x on if DEBUG is set to a non-empty string [ -n "$DEBUG" ] && set -x That way you can actually see what your script was doing right before it failed. Use trap for robust clean-ups A trap is a snippet of code that the shell executes when it exits or receives a signal. For example, pressing CTRL-C in the terminal where the script is running generates the INT signal. killing the process by default generates a TERM (I.e., terminate) signal. I find traps most useful for making sure my scripts clean-up after themselves whatever happens (e.g., a non-zero error code in -e mode). For example: #!/bin/sh -e TMPFILE=$(tempfile) trap 'echo "removing $TMPFILE"; rm -f $TMPFILE' INT TERM EXIT echo TMPFILE=$TMPFILE echo hello world > $TMPFILE cat $TMPFILE # gives user a chance to press CTRL-C sleep 3 # false always returns an error false echo "NEVER REACHED" 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: trap - INT TERM EXIT You can get future posts delivered by email or go
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