Download Linux Wrapper Script
Laurene Mallon
I need to make a bash script to replace a certain application.
Script should do some work and then execute said application in a way that everything (parameters both positional and named, env variables, signals etc.) will work as if it was the original application executed.
Try something similar for the security flag! See if your solution is similar to mine at the end of this tutorial when I post the entire script. Hint: you probably will want to use some global variable that you can change to fit into the yum check-update command that will be used later.
Basically, this puts the command output (of yum check-update) into an array (updateArr). Notice the reference to a CMD variable, which is another hint as to what you can do with the security argument from the beginning of the script.
So what do you think? Is it cool knowing that you are so confined to the default plugins that the ITL and 3rd parties provide? You can make your own check scripts now! All you have to do now is to call the script from Icinga.
The Wrapper shell script and batch file reside in the root directory of a single or multi-project Gradle build. You will need to reference the correct path to those files in case you want to execute the build from a subproject directory e.g. ../../gradlew tasks.
The better and recommended option is to run the wrapper task and provide the target Gradle version as described in Adding the Gradle Wrapper.Using the wrapper task ensures that any optimizations made to the Wrapper shell script or batch file with that specific Gradle version are applied to the project.
Note that running the wrapper task once will update gradle-wrapper.properties only, but leave the wrapper itself in gradle-wrapper.jar untouched.This is usually fine as new versions of Gradle can be run even with older wrapper files.
The Wrapper task fails if gradle-wrapper.properties contains distributionSha256Sum, but the task configuration does not define a sum.Executing the Wrapper task preserves the distributionSha256Sum configuration when the Gradle version does not change.
i had a problem. I have a debian linux machine here with an APC ups connected via usb cable running the lovely, small and simple apcupsd. Also another PC (running Windows) is connected to the same ups, so i installed apcupsd on windows too and connected it to the apcupsd on the linux machine, fine. Then i needed an additional NUT server so i can hook up my Synology NAS also to the same ups. But, trying to install the complicated nut server beside apcupsd on the same linux machine was a pain. So, i wrote this little script that emulates a nut-server and gets the values from apcupsd/apcaccess.
In particular, if the caller wants to kill the program, they'll just kill the process they just launched. If the wrapper script runs a child process, the caller would need to know that they should find out the child of the wrapper and kill that instead. The wrapper script could set a trap to relay some signals, but that wouldn't work with SIGSTOP or SIGKILL which can't be caught.
I solved this by writing one single wrapper for all executables, and a lot of symbolic links. This wrapper sets the environment variables for the relevant application, and then runs the desired executable. The path is set to run the wrapper, rather than the executable, so this is completely transparent. In this way, the new software sees the correct environment variables but without polluting them for the entire system.
Xilinx wanted me to add a lot of mumbo-jumbo into the .bashrc. Most went to the wrapper script (shown below). The only thing I put in .bashrc was appending a directory to the path. Xilinx wanted me to put $XILINX/bin/$PLATFORM, but I went for $XILINX/bin-wrappers/$PLATFORM
The idea is now to create a symbolic link for each executable in the bin directory, all pointing at xilinx-app-wrapper. This makes sense, since this script detects by which command it was called, and will exec the correct Xilinx application in turn.
Note that among all symlinks, we have xilinx-app-wrapper itself, which is the only thing that actually runs in this directory, and hence the only thing which needs changing when a Xilinx-global change in the environment is necessary.
I do notice that one of the functional test fails when I do a make check. Will this affect data being written to build-wrapper. I am trying to get the developers to look at this.
build-wrapperlog.txt (4.6 KB)
However, to avoid the long command flatpak run org.octave.Octave, I created a file .local/bin/octave with the contents flatpak run org.octave.Octave, and marked it as executable (I already removed the apt version of Octave). While it runs octave just fine, octave script.m does not work anymore, it only opens the command line version of Octave.
This tutorial is intended to do two things: to expand on the Cron Troubleshooting article; and to give an overview of a simple scripting concept that uses the creation of a file as a flag to signify something is running. This is primarily useful when you need to run something continuously, but not more than one copy at a time. You can create a file as a flag to check if a job is already running, , and in turn, check for that flag before taking further action.
The direct application of this is when you have a cron job that runs every minute, or every few minutes. With a rapidly repeating cron, if the previous job takes any longer than the scheduled time, these tasks can pile up causing load on the server, or exacerbating other issues. To avoid this, a simple script can be set up in the crontab (in place of the intended cron task). When the cron is run, it only runs the actual task if there is not a competing process already running.
A cron wrapper is used when you have a cron job that needs to run back to back but needs to not step on itself. This is good for tasks that you want to setup to run continuously. Jobs that should be run anywhere between every minute and every five minutes should be utilizing a wrapper like this.
The reason this is called a cron wrapper is that it is a script that wraps around the cron job, and checks if another instance of the cron is already running. If there is another copy running, the wrapper will make the cron skip this run, and wait until the next run to check again. There are a few ways that the cron wrappers ensures no overlap.
A straightforward method is to use what is called a lockfile. The cron wrapper checks if the lockfile (any file with a specific name/location) exists at the start of the run. If the lockfile is missing, the script creates that file and continues. The creation of the lockfile signals the start of the cron job. When the cron job completes the wrapper script then removes the lock file.
So long as the lockfile exists, a new wrapper will not run the rest of the cron job while another one is running. Once the first run completes and the lock is removed another wrapper will be able to create a new lock file again and process normally.
In the example above, it is convenient to define the lock file as a variable ($lockfile) so that it can be referenced easily later on. Also if you want to change the location, you only have to change it one place in the script.
Suppose you want to use GDB, the GNU debugger for C and C++ programs, to debug a program invoked from a shell script. You might have trouble knowing what is going on in the program because the script might give it a complicated run-time context, setting environment variables in various ways depending upon the machine, architecture, installed programs, etc. with which it's being run.
A good example of such a script is /usr/bin/firefox. On my Fedora 35 machine, the firefox script is 290 lines long. It mostly sets a lot of environment variables, but it also contains commands to make directories, remove files and directories, and make symbolic links. All these changes can have impacts on the binary when it runs. Near the end of the script, a command invokes (via exec) another script named run-mozilla.sh.
The run-mozilla.sh script itself is 356 lines long. It also sets environment variables and eventually invokes (also via exec) the Firefox binary. Additionally, the script provides options that allow you to debug the Firefox binary with a debugger, though for this article we won't use those options.
Use of a wrapper script to set environment variables and then invoke a binary is fairly common. On my Fedora 35 machine, more than 13 percent of the files in /usr/bin start with either #!/usr/bin/sh or #!/usr/bin/bash. An initial #! string on the first line of a text file is a convention known in Unix and Linux as a shebang. The line specifies the program that should run the script. In short, lots of programs in the directory named bin are not binaries. Some culminate in an exec command to run some other executable, as the run-mozilla.sh script does.
In the distant past, when attempting to debug programs associated with similar scripts, I'd examine the script and then set up what I perceived to be the relevant environment variables in an interactive shell session. After doing this, I'd invoke GDB in the usual way on the binary. However, it might take a fair amount of time to understand the wrapper script well enough to create an environment comparable to that created when running the script, and the whole procedure is error-prone.
It's common for wrapper scripts to use the shell's exec command to run a binary. The exec command causes the process in which the shell is running to be replaced by that of the binary. This is different from a fork and exec (which is used to run other non-builtin commands not prefixed by exec). A fork and exec creates a new process and enables the shell script to continue after the command it invokes has exited.
Make sure that the script in question uses exec to invoke the program you are debugging. You can identify whether the wrapper uses exec by simply searching for exec in the script. Once you find that command, verify that the exec command invokes the binary you want to debug. For instance, the last line of the /usr/bin/firefox script looks like this:
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