Ways to “monitor” a file in Linux

As I’ve been building out my production mail server, I had the need to monitor some files. Sure, I could have multiple windows/tabs open and click back and forth, or go re-run a command. However, this is Linux, surely there’s a way that I can monitor files in real-time? I just want the screen to update when the file has changed – but how do you do that?

In my case, I had two different scenarios:

  1. Monitor a file, and update the screen when more content was added to that file (like a log file)
  2. Monitor a file, and update the screen when any of the content has changed.

Below are some of the things I’ve learned.

Using “tail”:

The obvious answer for some is to use use “tail”. Tail is a command that will normally just show you the last 10 lines of a file and return. For example:

$ tail /var/log/syslog

You can change how many lines it returns, by using “-n 20” for example, to show the last 20 lines of a file. This is useful, but I don’t want to have to keep re-running that command. Well, there is a “-f” option, for “follow”. Meaning, instead of showing the tail of the file and returning to the prompt, a -f will hold the console open and update immediately if/when more content is written. You use it like this:

$ tail /var/log/syslog -f

and then use CTRL+C to cancel and go back to the command line.

Using “less”:

Another way to do this, but which gives you a little more functionality, is to use “less”. Less is typically use to let you scroll up and scroll down a text file in the console, but also lets you search it too. So, you could do this:

$ less /var/log/syslog

which let’s you page-up/page-down and search (at the : prompt, type “/” and then a search pattern). This is great, but the file might be getting bigger, and all I have is this snapshot of how it looked when I first loaded it. What you can do, is while in less, at the “:” prompt, type “F” (upper-case F), which will monitor the file for changes and show them as they are written to the file.

less-with-f

When you are in follow-mode (with “F”), you use CTRL+C to exit that mode, and you can type “q” to quit out of the less program.

What is good about this is that it does the same thing that “tail file -f” does, but also lets you page-up/page-down and search the contents too.

Using “watch”:

Now, the above are great for monitoring a log file like syslog. However, I had this other need where I wanted to monitor a small file and see the contents, as I changed them with a script I was testing. Although there might be a way to do that, I found this approach totally reasonable (and even ideal) – it’s called “watch”. Watch lets you run a command every n seconds, and shows the output on the screen. For example, if I wanted to see when files changed in a directory, I could run “ls -la” every few seconds. Here’s how you might use that:

$ watch -d -n 2 ls -la

the -d highlights things that are different on each refresh, and -n 2 says to refresh every 2 seconds. After that, is the command to run. That looks something like this:

watch

It’s tough to capture in a screenshot, but when i added “test3.txt”, it highlighted the file name to show that it’d changed. In my case, wanted to monitor the contents of a small file (something that fit on one screen), so as you might imagine, I could now accomplish that with:

$ watch -d -n 2 cat ~/somefile.txt

That outputs the contents of the file every 2 seconds, highlighting the changes. By the way, just in case you are not familiar – how do you know what all the command-line arguments are? Use “man”. For any command you type at the command line there is a “man page” for it. So, to learn more about watch, run:

$ man watch

That goes for any command that you run across on Linux.

Putting it together with “tmux”:

You might recall that I was enamored with this “tmux” tool I ran across. This ended up being a pretty good tool for the job. I ideally wanted to monitor syslog (because I was putting some messages there), and monitor the contents of the file which I was programatically modifying, and then I needed a place to type my commands – so I did something like this:

tmux-in-use

after running “tmux”, I did a CTRL+B, ” (that’s a CTRL+B, then a double-quote) to split the window horizontally. Then, I do CTRL+B, : to bring up the tmux input line, and I type “resize-pane -D 10” to reduce the size of that lower pane by 10. To navigate between panes, it’s CTRL+B, and then an arrow-key. So, CTRL+B, Up to go the next pane above. From there, I split the pane again horizontally. See that other blog post for more on how to use tmux. Finally, I had the 3 panes I wanted:

  • Top: where I could type in my commands
  • Middle: where I could monitor the contents of the file using “watch”
  • Bottom: where I could monitor syslog by using “tail -f”

In this particular case, I found it very frustrating to move between tabs and windows – particularly because I was working on another system via SSH. So, I would open a new window and start typing… “where did that file go!?” I would say, and then realize I was on the local system. When I’m SSH’ed into another system, I quickly forget – which frustrates me because I tend to open new tabs/windows all the time – never remembering that i need to re-SSH into that remote system in this window too.

Instead, in this particular scenario, just having everything in one screen which only took a few seconds to setup up, was ideal for me. This approach really took the frustration away (for me).

Posted in General, Linux, Uncategorized

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