Trending March 2024 # Ensure Only One Instance Of A Bash Script Is Running On Linux # Suggested April 2024 # Top 8 Popular

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Introduction

When running a bash script on Linux, it’s important to ensure that only one instance of the script is running at a time. This is especially important for scripts that perform critical tasks, such as scripts that update databases or scripts that send email. Running multiple instances of the same script simultaneously can cause conflicts, data loss, and other errors. In this article, we will discuss different methods to ensure that only one instance of a bash script runs on Linux.

Using Flock

One way to ensure that only one instance of a bash script runs on Linux is to use the “flock” command. The flock command is used to create a lock on a file and is a command built into most shells such as Bash, Zsh, etc. It’s a simple and efficient way to ensure that only one instance of a script runs at a time.

We can trust this approach because there will be no race conditions. Also, all locks on a file are released when the process completes. These benefits make flocking a safe way to ensure that only one instance is running. Another benefit is that the flock program is an implementation of the flock system call.

flock by default blocks until the lock is released and then continues without errors. We can use the “-n” parameter to use flock in a non-blocking way. This will cause flock to exit immediately with an error when there is another lock on the file.

We can use flock to run an external script or use it inside the script.

Running an External Script

We can use flock in a script like this −

$ flock -n chúng tôi ./dobackup.sh

Now let’s assume that our script is currently running. Let’s see what happens if we execute the previous line again −

$ flock --verbose -n chúng tôi ./dobackup.sh flock: failed to get lock $ echo $? 1

We can see that flock informed us that it could not acquire the lock and exited with the value 1 (error). This means that another instance has the lock.

When flock fails, it does not run the script parameter, which prevents more than one instance of “dobackup.sh” from running.

Using Flock within the Script

We can use flock inside the script like this −

#!/bin/bash another_instance() { echo "There is another instance running, exiting" exit 1 }

In this case, we call flock with a file descriptor and enclose everything we need to protect in square brackets (a subshell) and redirect it to the file we use as lock. We call flock at the beginning with the file descriptor used in the redirect. So if flock exits with an error, we know there is another instance running.

Once the sublevel is finished, the lock file is closed and the lock is automatically released.

Using the Pid File

Another way to ensure that only one instance of a bash script runs on Linux is to use a pid file. A pid file is a special file that contains the process ID (PID) of the running script. By checking the pid file, we can determine if the script is already running and if so, exit the script with an error message.

To implement this method, we can add the following code to the beginning of our script −

if [ -f /path/to/pidfile ]; then pid=$(cat /path/to/pidfile) echo "Script is already running" exit 1 else fi else fi

At the end of the script, you can add the following code to remove the pid file −

$ rm -f /path/to/pidfile Conclusion

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How To Run Bash Script As Root During Startup On Linux

Have you ever wanted to run a script at startup with root privileges? If you have a home server, or maybe even just a Linux desktop, this might have crossed your mind. This sounds iffy, but if you understand the risks, the reward for doing this can be quite good.

The main reasons are that there would be no more starting up the server, logging in over ssh, entering a password, getting a root shell and then manually executing script after script. Instead, harness the power of cron, and set your system to automatically run these scripts at startup! Here’s how to do it.

Tip: Check out our regular expressions cheatsheet.

Setting up Cron

Most Linux distributions come with the ability to access cron by just entering crontab -e. However, if you’ve entered this command, and nothing at all has happened, you’re on a Linux distribution that has no way to interact with cron. This means that you’ll need to install a tool to continue. The most popular tool to use in this situation is a daemon known as “cronie.” It’s a very popular tool and resides in most popular Linux distribution repositories.

Open up a terminal and install cronie with your package manager. Alternatively, head over to this page and download a package for your distribution.

Setting up the script with Cron

Opening a crontab is very easy. To start, open up a terminal window and enter the following command:

Note: the sudo is important if you want to run script as root. You can omit the sudo if you just want to run the script as a normal user.

If the system hasn’t used crontab before, the user will need to specify an editor to work with. Though all the editors are good in their own way, choose “nano” as it’s the simplest text editor and doesn’t require a lot of fussing with. With the editor selected, cron will load up a default file with detailed instructions as to how everything works.

Inside the nano editor in the terminal scroll all the way down to the bottom and start off by writing “@reboot.” The reboot command is key here as it tells the cron on reboot this command to run every single time. Directly after reboot, add the full file path to the bash script.

Now that the command is set up, the crontab can be saved. Press “Ctrl + o” on the keyboard. This will prompt the user to “write out the file.” By default, cron names the crontab, so don’t change anything. Press the enter key to save the crontab.

Remove the script from startup

In the same way that the command was added to the crontab, it can be removed. To do this, open up a terminal and enter sudo crontab -e. This will load the crontab file. Just delete the command that was added, save it, and restart the computer (or server).

Troubleshooting Cron

Sometimes cron doesn’t execute commands, and that can be a problem. The easiest way to troubleshoot any issues with cron (should there be any) is to look at the system log. To do that, open the terminal window and enter this command:

grep

CRON

/

var

/

log

/

syslog

The syslog shows all system events, and by using the grep command, it is possible to filter out what cron and crontag does. This should allow users to easily troubleshoot and fix anything that may go wrong.

Conclusion

Bash scripting is a wonderful thing, and its one of Linux’s great strengths. It makes administration of servers and even regular Linux computers easier because of the ability to take large amounts of commands and automate them. By adding cron to the picture, these scripts have the power to become even more useful. No more tinkering around after your Linux box boots. Just set it up and forget it!

What root scripts would you run at startup on your Linux box? Tell us below!

Derrik Diener

Derrik Diener is a freelance technology blogger.

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Running Script Or Command As Another User In Linux

There are several ways to run a script or command as another user in Linux. One way is to use the “su” command, which stands for “switch user.” For example, to run a command as the user “john,” you would use the following syntax: “su john -c ‘command'”

Another way to run a command as another user is to use the “sudo” command, which stands for “superuser do.” This command allows a user with proper permissions to run a command with the privileges of another user, typically the root user. For example, to run a command as the root user, you would use the following syntax: “sudo command”

You can also use the “runuser” command to run a command as another user, this command is similar to su command but it is more secure.

It’s important to note that using these commands can potentially be dangerous, as they allow a user to execute commands with escalated privileges. Therefore, it is important to use them with caution and only when necessary.

Using su

The “su” command is used to switch to another user’s account in Linux. The basic syntax for using the “su” command is as follows −

su [username]

For example, to switch to the “john” user, you would use the following command −

su john

When you run this command, you will be prompted to enter the password for the user you are switching to. Once you enter the correct password, you will be logged in as that user and will have the same privileges and permissions as that user.

You can also use the – option in order to switch to the previous user, for example −

su -

You can also run a command as another user by using the -c option followed by the command you want to run.

su john -c 'command'

It’s important to note that using “su” command can potentially be dangerous, as it allows a user to execute commands with escalated privileges. Therefore, it is important to use it with caution and only when necessary.

Using sudo

The “sudo” command is used to run a command with the privileges of another user, typically the root user, in Linux. The basic syntax for using the “sudo” command is as follows −

sudo [command]

For example, to run the “apt-get update” command as the root user, you would use the following command −

sudo apt-get update

When you run this command, you will be prompted to enter your own password. The system will then check if you have the necessary permissions to run the command as the root user. If you have the correct permissions, the command will be executed, otherwise, the command will be denied.

It’s important to note that running commands as the root user can be very dangerous if not done properly, as it allows a user to execute commands with escalated privileges. Therefore, it is important to use it with caution and only when necessary. And also, you should use sudo only for commands that require elevated privileges, not for every command.

Using runuser

The “runuser” command is used to run a command as another user in Linux, similar to the “su” command. The basic syntax for using the “runuser” command is as follows −

runuser [options] [username] [command]

For example, to run the command “ls -l” as the user “john”, you would use the following command −

runuser -l john -c 'ls -l'

The option -l is used to make the environment similar to a login shell and -c is used to specify the command that you want to run.

You can also use the -u option followed by the username to specify the user you want to run the command as −

runuser -u john ls -l

The “runuser” command is more secure than “su” command because it doesn’t start a shell and it doesn’t source any shell profile files which reduces the attack surface of the system.

It’s important to note that using the “runuser” command can potentially be dangerous, as it allows a user to execute commands with escalated privileges. Therefore, it is important to use it with caution and only when necessary.

Conclusion

There are several ways to run a script or command as another user in Linux, such as using the “su” command, the “sudo” command, or the “runuser” command. Each command has its own syntax and options, and it’s important to use them with caution and only when necessary.

The “su” command allows you to switch to another user’s account and run commands with that user’s privileges and permissions. The “sudo” command allows you to run a command with the privileges of the root user, but you need to have the correct permissions. And the “runuser” command allows you to run a command as another user, it is similar to su command but it is more secure as it doesn’t start a shell and it doesn’t source any shell profile files.

Solving The “Only One Message At A Time” Error On Chatgpt

ChatGPT is a remarkable AI chatbot that can generate remarkably detailed human-like written content. This powerful tool can understand real human language and provide users with insightful answers to their questions. However, when users attempt to send multiple messages simultaneously, ChatGPT displays an error message that says “Only one message at a time, Please wait for any other responses before sending another message, or wait one minute”. This article will provide insight into the cause of this error message and offer tips on how to resolve it.

The “Only one message at a time” error message occurs when a user tries to send multiple messages simultaneously on ChatGPT or other platforms. ChatGPT is designed to process one message at a time to ensure that the responses generated by the chatbot are accurate and relevant. This error message is implemented to prevent users from sending multiple messages at once, which can cause confusion and disrupt the conversation. One potential cause of this error message is a program that provides a Web.

See Also: What is Auto-GPT, and why does it matter?

If you encounter the “Only one message at a time” error message on ChatGPT, there are a few simple steps that you can take to fix it. First, wait for one response to complete before sending another message. This will ensure that ChatGPT has processed one message before you send another, preventing any errors from occurring. Additionally, you can try clearing your browser’s cache and cookies or use a different browser. If you have been inactive for too long, you must log out and log in to ChatGPT or refresh the page. If the problem persists, you can contact the help center at chúng tôi for further assistance.

The “Only one message at a time” error message is implemented in ChatGPT to prevent users from sending multiple messages simultaneously, which can cause confusion and disrupt the conversation. To avoid this error message, you should wait for one response to complete before sending another message. This will ensure that ChatGPT has processed one message before you send another, preventing any errors from occurring. Additionally, you can try clearing your browser’s cache and cookies or use a different browser. If you have been inactive for too long, you must log out and log in to ChatGPT or refresh the page. If the problem persists, you can contact the help center at chúng tôi for further assistance.

See More: Can Turnitin Detect Chat GPT?

The amount of time you should wait before sending another message on ChatGPT depends on various factors such as the number of users on the platform, the time of day, and the server capacity. According to some sources, wait times could be as little as a couple of minutes. Others suggest waiting for 15 to 30 minutes if you keep getting the same problem message no matter how many times you try to visit ChatGPT. It is recommended to use ChatGPT during off-peak hours to ensure an error-free experience. However, if you encounter the “Only one message at a time” error message, you should wait for one response to complete before sending another message to prevent any errors from occurring.

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Related

How To Block A Specific Instance On Mastodon

When you get started on Mastodon, you will find that a lot of its features resemble the ones you may find on Twitter. Like on Twitter, Mastodon also allows you to control what you see, what notifications you get, and who can communicate with you on the platform. If you find that a lot of people from a particular instance are annoying or posting problematic content, blocking them one by one may be a cumbersome task. 

To make it easier for you to control what you see on your home and public timelines, Mastodon offers a way to block an entire instance, so that people or posts from a particular instance no longer appear on your account. In this article, we’ll explain all the ways you can block a specific instance on Mastodon, what happens when you block an instance, and what you can do to view people and posts from a blocked instance again. 

When should you block an instance on Mastodon?

Like Twitter, Mastodon offers users the ability to avoid seeing content and interaction from people they don’t like on the platform. In addition to blocking and restricting individual accounts from appearing on your timeline or communicating with you, Mastodon also allows you to block entire instances and all of its users. 

When your home timeline is filled up with posts from another instance whose posts you don’t want to see or if this instance is full of people who post hateful or negative thoughts, then Mastodon provides a way for you to prevent this particular instance’s people or posts from ever appearing on your screen. For this, you will have to block an entire instance meaning no one from that instance will be able to contact you or see your posts on Mastodon. 

Related: How to find friends on Mastodon

How to block a specific instance on Mastodon

You can block specific domains or instances on Mastodon if you’re using the web client of the platform or your computer. Mastodon doesn’t let you block instances from its iOS or Android app, so you’ll have to make things do on the web when dealing with domain blocks. 

From someone’s Mastodon post

The selected domain will now be blocked from your account on Mastodon. 

Related: How to quote someone’s post on Mastodon

From someone’s profile

The selected domain will now be blocked from your account on Mastodon. 

Related: How to make your Mastodon account private

What happens when you block an instance on Mastodon?

When you block a specific instance or domain on Mastodon from your account:

You will no longer see posts from that instance on your home, local, or federated timelines. 

Your home feed won’t show posts from this instance if they were boosted by people or hashtags you followed or shared by people from within your instance. 

You will receive no notifications from that instance even if someone from that instance mentions you in their posts. 

You will lose all followers from the instance you just blocked. 

You won’t receive direct messages from someone who’s part of the blocked instance. 

Posts or replies where a person from the blocked instance is mentioned won’t show up on your home feed or other timelines. 

No one from the blocked instance will be able to follow you or send follow requests on Mastodon. 

If you think this is too extreme, you can opt to block certain people from an instance instead of blocking an entire domain on the platform. You can block anyone you find annoying or problematic on Mastodon by checking out the guide in the link below.

▶︎ How to block someone on Mastodon

How to find and unblock instances you’ve blocked on Mastodon

Blocking instances on Mastodon isn’t an irreversible process as you can still unblock them at a later time if you wish to interact with people in that instance. You will be able to find and unblock the servers you had previously blocked from your Mastodon instance on the web. Unfortunately, you cannot unblock instances when using the Mastodon app on our phone. 

From your Mastodon Profile

In the list of options that show up, select Blocked domains. 

When you do that, Mastodon will instantly unblock the selected domain. Posts from this instance will now start appearing on your timelines if someone your know mentions or boosts such posts. You will also be able to follow and get followed by people from this instance in the future. 

Using the search tool

When you do that, Mastodon will instantly unblock the selected domain. Posts from this instance will now start appearing on your timelines if someone your know mentions or boosts such posts. You will also be able to follow and get followed by people from this instance in the future.

Can you block and unblock instances on the Mastodon app?

No. Mastodon’s mobile app on iOS and Android lacks a lot of features that are available on its web client. While you can block and unblock people on the Mastodon app, there’s no option that lets you block an entire domain on the app. The app also doesn’t offer an unblock option that lets you unblock a domain you blocked on the web. 

Can you block an instance you’re a part of?

No. While you can block any instance on Mastodon, the option to block an instance is only available when you’re trying to block an instance that you’re not a part of. If you find users from within your own instance that are annoying or posting hateful thoughts or problematic content, you will only be able to block or report these individual accounts one by one by following the guide linked below. 

▶︎ How to block someone on Mastodon

If you find that you’re part of an instance where most of the users are posting the kind of content you like, you can move your account to another instance. This way, you can be part of another instance where you can interact with a different set of users. To move your account to another instance follow the guide in the link below. 

▶︎ How to switch servers on Mastodon: Everything you need to know

That’s all you need to know about blocking a specific instance on Mastodon. 

RELATED

Median In A Stream Of Integers (Running Integers) In C++

   and a min heap on right side to represent elements that are greater than effective median 2. After processing an incoming element, the number of elements in heaps differ utmost by 1 element 3. When both heaps contain same number of elements, we pick average of heaps root data as effective median using namespace std; #define MAX_HEAP_SIZE (128) #define ARRAY_SIZE(a) sizeof(a)/sizeof(a[0]) inline void Exch(int &a, int &b){    int aux = a;    a = b;    b = aux; } bool Greater(int a, int b){ } bool Smaller(int a, int b){    return a < b; } int Average(int a, int b){    return (a + b) / 2; } int Signum(int a, int b){    if( a == b ) {       return 0;    }    return a < b ? -1 : 1; } class Heap{    public:       Heap(int *b, bool (*c)(int, int)) : A(b), comp(c){          heapSize = -1;       }       virtual ~Heap(){          if( A ) {             delete[] A;          }       }       virtual bool Insert(int e) = 0;       virtual int GetTop() = 0;       virtual int ExtractTop() = 0;       virtual int GetCount() = 0;       protected:       int left(int i){          return 2 * i + 1;       }       int right(int i){          return 2 * (i + 1);       }       int parent(int i){          if( i <= 0 ) {             return -1;          }          return (i – 1)/2;       }       int *A;       bool (*comp)(int, int);       int heapSize;       int top(void){          int max = -1;             max = A[0];          }          return max;       }       int count(){          return heapSize + 1;       }       void heapify(int i){          int p = parent(i);             Exch(A[i], A[p]);             heapify(p);          }       }       int deleteTop(){          int del = -1;             del = A[0];             Exch(A[0], A[heapSize]);             heapSize–;             heapify(parent(heapSize+1));          }          return del;       }       bool insertHelper(int key){          bool ret = false;          if( heapSize < MAX_HEAP_SIZE ) {             ret = true;             heapSize++;             A[heapSize] = key;             heapify(heapSize);          }          return ret;       } }; class MaxHeap : public Heap{ private: public:    MaxHeap() : Heap(new int[MAX_HEAP_SIZE], &Greater) { }    ~MaxHeap() { }    int GetTop(){       return top();    }    int ExtractTop(){       return deleteTop();    }    int GetCount(){       return count();    }    bool Insert(int key){       return insertHelper(key);    } }; class MinHeap : public Heap{ private: public:    MinHeap() : Heap(new int[MAX_HEAP_SIZE], &Smaller) { }    ~MinHeap() { }    int GetTop(){       return top();    }    int ExtractTop(){       return deleteTop();    }    int GetCount(){       return count();    }    bool Insert(int key){       return insertHelper(key);    } }; int getMedian(int e, int &m, Heap &l, Heap &r){    int sig = Signum(l.GetCount(), r.GetCount());    switch(sig){       case 1:          if( e < m ) {             r.Insert(l.ExtractTop());             l.Insert(e);          } else {             r.Insert(e);          }          m = Average(l.GetTop(), r.GetTop());          break;          case 0:          if( e < m ) {             l.Insert(e);             m = l.GetTop();          } else {             r.Insert(e);             m = r.GetTop();          }          break;          case -1:          if( e < m ) {             l.Insert(e);          } else {             l.Insert(r.ExtractTop());             r.Insert(e);          }          m = Average(l.GetTop(), r.GetTop());          break;       }    return m; } void printMedian(int A[], int size){    int m = 0;    Heap *left = new MaxHeap();    Heap *right = new MinHeap();    for(int i = 0; i < size; ++i) {       m = getMedian(A[i], m, *left, *right);       cout << m << endl;    }    delete left;    delete right; } int main(){    int A[] = {10, 20, 30};    int size = ARRAY_SIZE(A);    cout “Result:n”;    printMedian(A, size);    return 0; }

Output

When you compile and execute the above program. It generates the following output −

Result: 10 15 20

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