how to use the output of a bash script in a Golang function

Question

This might not even be possible but I've been researching for the past hour and come up blank. I have a bash script that gives me a certain output and I want to add that output to Golang in order to redirect a website based on the output of the bash script. Sorry if this makes no sense, im new to Go

Here is what I have to run the bash script and output the value

 func main() {
    out, err := exec.Command("/bin/sh", "script.sh").Output()
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf(string(out))
}

I then want to use the value that was output there in another function and to redirect a URL heres how I would redirect to a url and I wanted the $variable to be added. This is just an example I copied off the internet but its what I want to replicate.

func redirect(w http.ResponseWriter, r *http.Request) {

    http.Redirect(w, r, "**$variable**", 301)
}
 func main() {
     http.HandleFunc("/", redirect)
     err := http.ListenAndServe(":8080", nil)
     if err != nil {
         log.Fatal("ListenAndServe: ", err)
     }
 }

At what moment is your bash script run? At startup?

Benjamin Barrois
– Benjamin Barrois

2022-01-11 18:50:36 +00:00
Commented Jan 11, 2022 at 18:50 — Benjamin Barrois
– Benjamin Barrois, Commented Jan 11, 2022 at 18:50

Benjamin Barrois · Accepted Answer · 2022-01-14 16:43:21Z

1

Assuming your script must be only run once at startup, then this should do the trick:

var redirectTo string

func redirect(w http.ResponseWriter, r *http.Request) {
    http.Redirect(w, r, redirectTo, 301)
}

func main() {
    out, err := exec.Command("/bin/sh", "script.sh").Output()
    if err != nil {
       log.Fatal(err)
    }
    redirectTo = string(out)
    http.HandleFunc("/", redirect)
    err = http.ListenAndServe(":8080", nil)
    if err != nil {
        log.Fatal("ListenAndServe: ", err)
    }
}

Or if you don't want to have a global variable you can generate the redirect function at runtime:

func main() {
    out, err := exec.Command("/bin/sh", "script.sh").Output()
    if err != nil {
       log.Fatal(err)
    }
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        http.Redirect(w, r, string(out), 301)
    })
    err = http.ListenAndServe(":8080", nil)
    if err != nil {
        log.Fatal("ListenAndServe: ", err)
    }
}

if you want to handle when out is empty, you can do:

func redirect(w http.ResponseWriter, r *http.Request) {
    if len(redirectTo) == 0 {
        http.Error(w, "No URL parsed!", 500)
        return
    }
    http.Redirect(w, r, redirectTo, 301)
}

This will return an HTTP error 500 in that case.

You can also simply exit the program instead of serving:

out, err := exec.Command("/bin/sh", "script.sh").Output()
if err != nil {
   log.Fatal(err)
}
if len(out) == 0 {
   log.Fatal("no output was returned from script")
}

You can also add more verifications to out here if you wish, like if it is a correct URL, using net/url package for instance.

edited Jan 14, 2022 at 16:43

answered Jan 11, 2022 at 18:52

Benjamin Barrois

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6 Comments

user5728991 Over a year ago

If string(out) has trailing whitespace, then OP will want redirectTo = strings.TrimSpace(string(out)).

Benjamin Barrois Over a year ago

Yes you may do that, it depends on if you are certain your script will not add trailing spaces, tabs or \n for instance

user17733554 Over a year ago

When I ran both your solutions I got an error "no new variables on left side of :=" which was pointing to the second err := I saw a solution saying to remove the colon for the second err := Would that cause any issues if I removed it? Other than that your solution worked exactly as I wanted, once I removed that colon. @BenjaminBarrois

user17733554 Over a year ago

@BenjaminBarrois im now getting this error when I open the webpage ERR_TOO_MANY_REDIRECTS: localhost redirected you too many times. is there a way to modify the script to only redirect once it has the out variable? or just a way to avoid that error

Benjamin Barrois Over a year ago

You are right I replaced the := by =, because err variable is already declared (sorry I had not tested my code).

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