Use exec to Redirect stdio in a git Hook Script
Redirection like ls > out only applies to a single line, so how do we control bash's input and output streams for blocks of commands? exec does just that -- it changes where stdout and stderr go for all commands that come after it.
In this lesson, we'll use exec in a script to redirect all stdout and stderr to a log file. Our script will be a post-merge git hook, which runs after a git pull is performed. It'll check git's list of changed files and run npm install if package.json was changed. Because git operations are sometimes performed inside UI instead of the shell directly, it can be handy to capture any errors in a log file.
Instructor: [00:00] First, let's set up our exec statement. I'm going to do exec, and I'm going to tell it to append standard out to log/hooks, out stlog. You can see, I have this log folder set up, and in this script, I want all of the standard out to go to this file, which will be created.
[00:18] I'm also going to tell it to send standard error to the same log file. Note that we're telling standard out to append to this file. That's what the double angle bracket says. We don't need that here. This is telling it to send standard error to the same place and in the same mode that standard out is in. Standard error will append to this file as well.
[00:39] Exec makes sense in this scenario, because the script won't be executed directly by our user. It'll be executed by Git. If we're using a Git UI client, we may have different environment variables, which can lead to unusual errors. We may not have easy access to view its output. It can be helpful to send it to a log file to debug it, or just make sure it's working.
[00:59] Now, let's check of our package.json has changed. We're going to do an if statement here, and we're going to run the git diff tree command. This compares two subtrees of Git. We'll do some formatting here.
[01:14] This tells it to compare the previous commit to the current commit that was just pulled. This will return just a bare list of files that have changed between the two commits, or between the two working trees. Then from there, we're going to pipe that to grep.
[01:32] I'm going to tell grep to not output anything. I'm going to use the quiet flag, and I'm going to grep for package.json. This if conditional syntax here, basically, this is saying if this command returns an exit status of zero, then this evaluates to true. If it's above zero, then it'll evaluate to false.
[01:54] The important part to remember here is that grep, if it finds package.json, the exit status of grep will be zero. If it doesn't find it, it'll be one. That's really what we're leveraging here to check. In here, this is where we did find it.
[02:07] I'm going to have some output here. I'm going to output the date, and I'll say running npm install. Then I'll run npm install here. Don't do else. No changes in package.json found. At our npm install here, npm install be a little bit verbose, and output a lot.
[02:26] I'm going to tell it to redirect its standard out to the device null file, or dev null. This is a common idiom in Bash, where you redirect standard out to dev null. Basically, that just says discard any of the output.
[02:41] It doesn't get to sent to a file. It doesn't show up on the screen or anything. It just silences it. This is useful for us here, because we don't want our log file getting cluttered up with the output of npm install. However, note that standard error, though, is not redirected here at all.
[02:58] If npm install errors out, and has some standard error output, that'll get captured in our log file. Save that. Let's jump back over here to our terminal. First, let's make sure that our script has execute permission.
[03:12] Now, let's link our Git hook into the right location. We use the link command. This is our file that we created. Then we'll pass the destination. The .git/hooks folder, that's where all Git hooks live. Our Git hook has to be in this folder to be run.
[03:30] Let's run that. We can see that that linked it there. Let's just go ahead and commit this really fast, and push it. Then over here, I have the same repository pulled, just in a separate folder. We'll go ahead and pull.
[03:47] Then we will make a change to the package.json. I'll just add a dependency. I'll do git commit, and then I'll do a push. Now, back in our original folder, where we set up our Git hook script, if I run Git pull, it should run our script.
[04:02] Let's check our log file. We can see it ran. Here, we outputted the date, and we say running npm install, because package.json changed. Awesome. It's working.