📚 script - Awesome Go Library for Software Packages

Go Gopher mascot for script

Making it easy to write shell-like scripts in Go for DevOps and system administration tasks.

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📂 Software written in Go.
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Detailed Description of script

Go Reference Go Report Card Mentioned in Awesome Go CI Audit

import "github.com/bitfield/script"

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What is script?

script is a Go library for doing the kind of tasks that shell scripts are good at: reading files, executing subprocesses, counting lines, matching strings, and so on.

Why shouldn't it be as easy to write system administration programs in Go as it is in a typical shell? script aims to make it just that easy.

Shell scripts often compose a sequence of operations on a stream of data (a pipeline). This is how script works, too.

This is one absolutely superb API design. Taking inspiration from shell pipes and turning it into a Go library with syntax this clean is really impressive.
Simon Willison

Read more: Scripting with Go

Quick start: Unix equivalents

If you're already familiar with shell scripting and the Unix toolset, here is a rough guide to the equivalent script operation for each listed Unix command.

Unix / shellscript equivalent
(any program name)Exec
[ -f FILE ]IfExists
>WriteFile
>>AppendFile
$*Args
base64DecodeBase64 / EncodeBase64
basenameBasename
catFile / Concat
curlDo / Get / Post
cutColumn
dirnameDirname
echoEcho
findFindFiles
grepMatch / MatchRegexp
grep -vReject / RejectRegexp
headFirst
jqJQ
lsListFiles
sedReplace / ReplaceRegexp
sha256sumHash / HashSums
tailLast
teeTee
uniq -cFreq
wc -lCountLines
xargsExecForEach

Some examples

Let's see some simple examples. Suppose you want to read the contents of a file as a string:

contents, err := script.File("test.txt").String()

That looks straightforward enough, but suppose you now want to count the lines in that file.

numLines, err := script.File("test.txt").CountLines()

For something a bit more challenging, let's try counting the number of lines in the file that match the string Error:

numErrors, err := script.File("test.txt").Match("Error").CountLines()

But what if, instead of reading a specific file, we want to simply pipe input into this program, and have it output only matching lines (like grep)?

script.Stdin().Match("Error").Stdout()

Just for fun, let's filter all the results through some arbitrary Go function:

script.Stdin().Match("Error").FilterLine(strings.ToUpper).Stdout()

That was almost too easy! So let's pass in a list of files on the command line, and have our program read them all in sequence and output the matching lines:

script.Args().Concat().Match("Error").Stdout()

Maybe we're only interested in the first 10 matches. No problem:

script.Args().Concat().Match("Error").First(10).Stdout()

What's that? You want to append that output to a file instead of printing it to the terminal? You've got some attitude, mister. But okay:

script.Args().Concat().Match("Error").First(10).AppendFile("/var/log/errors.txt")

And if we'd like to send the output to the terminal as well as to the file, we can do that:

script.Echo("data").Tee().AppendFile("data.txt")

We're not limited to getting data only from files or standard input. We can get it from HTTP requests too:

script.Get("https://wttr.in/London?format=3").Stdout()
// Output:
// London: 🌦   +13°C

That's great for simple GET requests, but suppose we want to send some data in the body of a POST request, for example. Here's how that works:

script.Echo(data).Post(URL).Stdout()

If we need to customise the HTTP behaviour in some way, such as using our own HTTP client, we can do that:

script.NewPipe().WithHTTPClient(&http.Client{
	Timeout: 10 * time.Second,
}).Get("https://example.com").Stdout()

Or maybe we need to set some custom header on the request. No problem. We can just create the request in the usual way, and set it up however we want. Then we pass it to Do, which will actually perform the request:

req, err := http.NewRequest(http.MethodGet, "http://example.com", nil)
req.Header.Add("Authorization", "Bearer "+token)
script.Do(req).Stdout()

The HTTP server could return some non-okay response, though; for example, “404 Not Found”. So what happens then?

In general, when any pipe stage (such as Do) encounters an error, it produces no output to subsequent stages. And script treats HTTP response status codes outside the range 200-299 as errors. So the answer for the previous example is that we just won't see any output from this program if the server returns an error response.

Instead, the pipe “remembers” any error that occurs, and we can retrieve it later by calling its Error method, or by using a sink method such as String, which returns an error value along with the result.

Stdout also returns an error, plus the number of bytes successfully written (which we don't care about for this particular case). So we can check that error, which is always a good idea in Go:

_, err := script.Do(req).Stdout()
if err != nil {
	log.Fatal(err)
}

If, as is common, the data we get from an HTTP request is in JSON format, we can use JQ queries to interrogate it:

data, err := script.Do(req).JQ(".[0] | {message: .commit.message, name: .commit.committer.name}").String()

We can also run external programs and get their output:

script.Exec("ping 127.0.0.1").Stdout()

Note that Exec runs the command concurrently: it doesn't wait for the command to complete before returning any output. That's good, because this ping command will run forever (or until we get bored).

Instead, when we read from the pipe using Stdout, we see each line of output as it's produced:

PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.056 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.054 ms
...

In the ping example, we knew the exact arguments we wanted to send the command, and we just needed to run it once. But what if we don't know the arguments yet? We might get them from the user, for example.

We might like to be able to run the external command repeatedly, each time passing it the next line of data from the pipe as an argument. No worries:

script.Args().ExecForEach("ping -c 1 {{.}}").Stdout()

That {{.}} is standard Go template syntax; it'll substitute each line of data from the pipe into the command line before it's executed. You can write as fancy a Go template expression as you want here (but this simple example probably covers most use cases).

If there isn't a built-in operation that does what we want, we can just write our own, using Filter:

script.Echo("hello world").Filter(func (r io.Reader, w io.Writer) error {
	n, err := io.Copy(w, r)
	fmt.Fprintf(w, "\nfiltered %d bytes\n", n)
	return err
}).Stdout()
// Output:
// hello world
// filtered 11 bytes

The func we supply to Filter takes just two parameters: a reader to read from, and a writer to write to. The reader reads the previous stages of the pipe, as you might expect, and anything written to the writer goes to the next stage of the pipe.

If our func returns some error, then, just as with the Do example, the pipe's error status is set, and subsequent stages become a no-op.

Filters run concurrently, so the pipeline can start producing output before the input has been fully read, as it did in the ping example. In fact, most built-in pipe methods, including Exec, are implemented using Filter.

If we want to scan input line by line, we could do that with a Filter function that creates a bufio.Scanner on its input, but we don't need to:

script.Echo("a\nb\nc").FilterScan(func(line string, w io.Writer) {
	fmt.Fprintf(w, "scanned line: %q\n", line)
}).Stdout()
// Output:
// scanned line: "a"
// scanned line: "b"
// scanned line: "c"

And there's more. Much more. Read the docs for full details, and more examples.

A realistic use case

Let's use script to write a program that system administrators might actually need. One thing I often find myself doing is counting the most frequent visitors to a website over a given period of time. Given an Apache log in the Common Log Format like this:

212.205.21.11 - - [30/Jun/2019:17:06:15 +0000] "GET / HTTP/1.1" 200 2028 "https://example.com/ "Mozilla/5.0 (Linux; Android 8.0.0; FIG-LX1 Build/HUAWEIFIG-LX1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.156 Mobile Safari/537.36"

we would like to extract the visitor's IP address (the first column in the logfile), and count the number of times this IP address occurs in the file. Finally, we might like to list the top 10 visitors by frequency. In a shell script we might do something like:

cut -d' ' -f 1 access.log |sort |uniq -c |sort -rn |head

There's a lot going on there, and it's pleasing to find that the equivalent script program is quite brief:

package main

import (
	"github.com/bitfield/script"
)

func main() {
	script.Stdin().Column(1).Freq().First(10).Stdout()
}

Let's try it out with some sample data:

16 176.182.2.191
 7 212.205.21.11
 1 190.253.121.1
 1 90.53.111.17

Documentation

See pkg.go.dev for the full documentation, or read on for a summary.

Sources

These are functions that create a pipe with a given contents:

SourceContents
Argscommand-line arguments
DoHTTP response
Echoa string
Execcommand output
Filefile contents
FindFilesrecursive file listing
GetHTTP response
IfExistsdo something only if some file exists
ListFilesfile listing (including wildcards)
PostHTTP response
Sliceslice elements, one per line
Stdinstandard input

Modifiers

These are methods on a pipe that change its configuration:

SourceModifies
WithEnvenvironment for commands
WithErrorpipe error status
WithHTTPClientclient for HTTP requests
WithReaderpipe source
WithStderrstandard error output stream for command
WithStdoutstandard output stream for pipe

Filters

Filters are methods on an existing pipe that also return a pipe, allowing you to chain filters indefinitely. The filters modify each line of their input according to the following rules:

FilterResults
Basenameremoves leading path components from each line, leaving only the filename
ColumnNth column of input
Concatcontents of multiple files
DecodeBase64input decoded from base64
Dirnameremoves filename from each line, leaving only leading path components
Doresponse to supplied HTTP request
Echoall input replaced by given string
EncodeBase64input encoded to base64
Execfiltered through external command
ExecForEachexecute given command template for each line of input
Filteruser-supplied function filtering a reader to a writer
FilterLineuser-supplied function filtering each line to a string
FilterScanuser-supplied function filtering each line to a writer
Firstfirst N lines of input
Freqfrequency count of unique input lines, most frequent first
Getresponse to HTTP GET on supplied URL
HashSumshashes of each listed file
Joinreplace all newlines with spaces
JQresult of jq query
Lastlast N lines of input
Matchlines matching given string
MatchRegexplines matching given regexp
Postresponse to HTTP POST on supplied URL
Rejectlines not matching given string
RejectRegexplines not matching given regexp
Replacematching text replaced with given string
ReplaceRegexpmatching text replaced with given string
Teeinput copied to supplied writers

Note that filters run concurrently, rather than producing nothing until each stage has fully read its input. This is convenient for executing long-running commands, for example. If you do need to wait for the pipeline to complete, call Wait.

Sinks

Sinks are methods that return some data from a pipe, ending the pipeline and extracting its full contents in a specified way:

SinkDestinationResults
AppendFileappended to file, creating if it doesn't existbytes written, error
Bytesdata as []byte, error
Hashhash, error
CountLinesnumber of lines, error
Readgiven []bytebytes read, error
Slicedata as []string, error
Stdoutstandard outputbytes written, error
Stringdata as string, error
Waiterror
WriteFilespecified file, truncating if it existsbytes written, error

What's new

VersionNew
0.23.0WithEnv
DecodeBase64 / EncodeBase64
Wait returns error
v0.22.0Tee, WithStderr
v0.21.0HTTP support: Do, Get, Post
v0.20.0JQ

Contributing

See the contributor's guide for some helpful tips if you'd like to contribute to the script project.

Links

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