Replace a line in text file Golang - go

How do I replace a line in a text file with a new line?
Assume I've opened the file and have every line in an array of string objects i'm now looping through
//find line with ']'
for i, line := range lines {
if strings.Contains(line, ']') {
//replace line with "LOL"
?
}
}

What matters here is not so much what you do in that loop. It's not like you're gonna be directly editing the file on the fly.
The most simple solution for you is to just replace the string in the array and then write the contents of the array back to your file when you're finished.
Here's some code I put together in a minute or two. It properly compiles and runs on my machine.
package main
import (
"io/ioutil"
"log"
"strings"
)
func main() {
input, err := ioutil.ReadFile("myfile")
if err != nil {
log.Fatalln(err)
}
lines := strings.Split(string(input), "\n")
for i, line := range lines {
if strings.Contains(line, "]") {
lines[i] = "LOL"
}
}
output := strings.Join(lines, "\n")
err = ioutil.WriteFile("myfile", []byte(output), 0644)
if err != nil {
log.Fatalln(err)
}
}
There's a gist too (with the same code)
https://gist.github.com/dallarosa/b58b0e3425761e0a7cf6

Related

Print new line from argument in command line

Morning All
I am trying to put together an ASCII art generator which accepts a word as a command line argument and prints out in ASCII art. This is working so far however, instead of interpreting "\n" as a new line it is printing these characters as ASCII art as well. Any ideas on how I can print a new line instead?
I have tried splitting the argument using code below but doesn't seem to do anything:
split := strings.Split(wordArg[1], "\n")
fmt.Println(split)
This just prints out [hello\n].
I am printing using printf if there is anything I can add there to help interpret the \n as new line instead of as being part of the string.
I have added the whole code if it helps make sense of what I'm doing:
package main
import (
"bufio"
"fmt"
//"strings"
//"io/ioutil"
"log"
"os"
"strings"
)
func main() {
wordArg := os.Args
split := strings.Split(wordArg[1], "\n")
fmt.Println(split)
wordRune := []rune(wordArg[1])
f, err := os.Open("standard.txt")
if err != nil {
fmt.Println(err)
}
defer f.Close()
scanner := bufio.NewScanner(f)
var lines []string
for scanner.Scan() {
lines = append(lines, scanner.Text())
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
for i := 0; i < 8; i++ {
for j := 0; j < len(wordRune); j++ {
if lines[int(wordRune[j])*9-287+i] == " " {
fmt.Printf(" ")
} else {
fmt.Printf(lines[int(wordRune[j])*9-287+i])
}
}
fmt.Print("\n")
}
}
Any help would be much appreciated.
Thanks
strings.ReplaceAll
func ReplaceAll(str, oldstr, newstr string) string
The Replace All function does as the name suggests, and replaces all occurrences of oldstr with newstr within str.
strings.ReplaceAll(os.Args[1], "\\n", "\n")
This will return a string containing the first argument from the command line, with newlines in place of a literal backslash and letter n.

Go regExRepl-Script does not change the text file

my go script should add one newline before matching the regEx-Search-String ^(.+[,]+\n).
The Prototype i had tested before into the editor:
i want add newlines before this lines: \n$1.
This works if i try it into the Text-Editor.
If i try this (see line 24) with my script it is changing nothing and sends no error.
Any ideas what i do wrong?
Example
i like to use PCRE like it works in this Example https://regex101.com/r/sB9wW6/17
Same Example here:
Example source
Dear sir,
Thanks for your interest.
expected result
#### here is a newline ####
Dear sir,
Thanks for your interest.
result is (produced by the script below)
Dear sir,
Thanks for your interest.
go script:
// replace in files and store the new copy of it.
package main
import (
"fmt"
"io/ioutil"
"os"
"path/filepath"
"regexp"
"strings"
"time"
)
func visit(path string, fi os.FileInfo, err error) error {
matched, err := filepath.Match("*.csv", fi.Name())
if err != nil {
panic(err)
return err
}
if matched {
read, err := ioutil.ReadFile(path)
if err != nil {
panic(err)
}
newContents := string(read)
newContents = regExRepl(`^(.+[,]+\n)`, newContents, `\n\n\n$1`)
var re = regexp.MustCompile(`[\W]+`)
t_yymmdd := regexp.MustCompile(`[\W]+`).ReplaceAllString(time.Now().Format(time.RFC3339), `-`)[:10]
t_hhss := re.ReplaceAllString(time.Now().Format(time.RFC3339), `-`)[11:19]
t_yymmddhhss := t_yymmdd + "_" + t_hhss
fmt.Println(t_yymmddhhss)
filePath := fileNameWithoutExtension(path) + t_yymmddhhss + ".csv"
err = ioutil.WriteFile(filePath, []byte(newContents), 0)
if err != nil {
panic(err)
}
}
return nil
}
func regExRepl(regExPatt string, newContents string, regExRepl string) string {
return regexp.MustCompile(regExPatt).ReplaceAllString(newContents, regExRepl)
}
func main() {
err := filepath.Walk("./november2020messages.csv", visit) // <== read all files in current folder 20:12:06 22:44:42
if err != nil {
panic(err)
}
}
func fileNameWithoutExtension(fileName string) string {
return strings.TrimSuffix(fileName, filepath.Ext(fileName))
}
for interpretation \n as newline don't us
`\n`` use "\n"
may use ^(.+[,]+) instead ^(.+[,]+\n) and ad (?m) before for multi-line replacements
this suggestion you could test here: https://play.golang.org/p/25_0GJ93oCT
The following example illustrates the difference (in golang-playground here https://play.golang.org/p/FkPwElhx-Xu ):
// example from:
package main
import (
"fmt"
"regexp"
)
func main() {
newContents := `line 1,
line 2
line a,
line b`
newContents1 := regexp.MustCompile(`^(.+[,]+\n)`).ReplaceAllString(newContents, `\n$1`)
fmt.Println("hi\n" + newContents1)
newContents1 = regexp.MustCompile(`(?m)^(.+[,]+\n)`).ReplaceAllString(newContents, "\n$1")
fmt.Println("ho\n" + newContents1)
}
Result:
hi
\nline 1,
line 2
line a,
line b
ho
line 1,
line 2
line a,
line b

Replacing a line within a file with Golang

I'm new to Golang, starting out with some examples. Currently, what I'm trying to do is reading a file line by line and replace it with another string in case it meets a certain condition.
The file is use for testing purposes contains four lines:
one
two
three
four
The code working on that file looks like this:
func main() {
file, err := os.OpenFile("test.txt", os.O_RDWR, 0666)
if err != nil {
panic(err)
}
reader := bufio.NewReader(file)
for {
fmt.Print("Try to read ...\n")
pos,_ := file.Seek(0, 1)
log.Printf("Position in file is: %d", pos)
bytes, _, _ := reader.ReadLine()
if (len(bytes) == 0) {
break
}
lineString := string(bytes)
if(lineString == "two") {
file.Seek(int64(-(len(lineString))), 1)
file.WriteString("This is a test.")
}
fmt.Printf(lineString + "\n")
}
file.Close()
}
As you can see in the code snippet, I want to replace the string "two" with "This is a test" as soon as this string is read from the file.
In order to get the current position within the file I use Go's Seek method.
However, what happens is that always the last line gets replaced by This is a test, making the file looking like this:
one
two
three
This is a test
Examining the output of the print statement which writes the current file position to the terminal, I get that kind of output after the first line has been read:
2016/12/28 21:10:31 Try to read ...
2016/12/28 21:10:31 Position in file is: 19
So after the first read, the position cursor already points to the end of my file, which explains why the new string gets appended to the end. Does anyone understand what is happening here or rather what is causing that behavior?
The Reader is not controller by the file.Seek. You have declared the reader as: reader := bufio.NewReader(file) and then you read one line at a time bytes, _, _ := reader.ReadLine() however the file.Seek does not change the position that the reader is reading.
Suggest you read about the ReadSeeker in the docs and switch over to using that. Also there is an example using the SectionReader.
Aside from the incorrect seek usage, the difficulty is that the line you're replacing isn't the same length as the replacement. The standard approach is to create a new (temporary) file with the modifications. Assuming that is successful, replace the original file with the new one.
package main
import (
"bufio"
"io"
"io/ioutil"
"log"
"os"
)
func main() {
// file we're modifying
name := "text.txt"
// open original file
f, err := os.Open(name)
if err != nil {
log.Fatal(err)
}
defer f.Close()
// create temp file
tmp, err := ioutil.TempFile("", "replace-*")
if err != nil {
log.Fatal(err)
}
defer tmp.Close()
// replace while copying from f to tmp
if err := replace(f, tmp); err != nil {
log.Fatal(err)
}
// make sure the tmp file was successfully written to
if err := tmp.Close(); err != nil {
log.Fatal(err)
}
// close the file we're reading from
if err := f.Close(); err != nil {
log.Fatal(err)
}
// overwrite the original file with the temp file
if err := os.Rename(tmp.Name(), name); err != nil {
log.Fatal(err)
}
}
func replace(r io.Reader, w io.Writer) error {
// use scanner to read line by line
sc := bufio.NewScanner(r)
for sc.Scan() {
line := sc.Text()
if line == "two" {
line = "This is a test."
}
if _, err := io.WriteString(w, line+"\n"); err != nil {
return err
}
}
return sc.Err()
}
For more complex replacements, I've implemented a package which can replace regular expression matches. https://github.com/icholy/replace
import (
"io"
"regexp"
"github.com/icholy/replace"
"golang.org/x/text/transform"
)
func replace2(r io.Reader, w io.Writer) error {
// compile multi-line regular expression
re := regexp.MustCompile(`(?m)^two$`)
// create replace transformer
tr := replace.RegexpString(re, "This is a test.")
// copy while transforming
_, err := io.Copy(w, transform.NewReader(r, tr))
return err
}
OS package has Expand function which I believe can be used to solve similar problem.
Explanation:
file.txt
one
two
${num}
four
main.go
package main
import (
"fmt"
"os"
)
var FILENAME = "file.txt"
func main() {
file, err := os.ReadFile(FILENAME)
if err != nil {
panic(err)
}
mapper := func(placeholderName string) string {
switch placeholderName {
case "num":
return "three"
}
return ""
}
fmt.Println(os.Expand(string(file), mapper))
}
output
one
two
three
four
Additionally, you may create a config (yml or json) and
populate that data in the map that can be used as a lookup table to store placeholders as well as their replacement strings and modify mapper part to use this table to lookup placeholders from input file.
e.g map will look like this,
table := map[string]string {
"num": "three"
}
mapper := func(placeholderName string) string {
if val, ok := table[placeholderName]; ok {
return val
}
return ""
}
References:
os.Expand documentation: https://pkg.go.dev/os#Expand
Playground

How to read/scan a .txt file in GO

The .txt file has many lines which each contain a single word. So I open the file and pass it to the reader:
file, err := os.Open("file.txt")
check(err)
reader := bufio.NewReader(file)
Now I want to store each line in a slice of strings. I believe I need to use ReadBytes, ReadString, ReadLine, or on of the Scan functions. Any advice on how to implement this would be appreciated. Thanks.
You can use ioutil.ReadFile() to read all lines into a byte slice and then call split on the result:
package main
import (
"fmt"
"io/ioutil"
"log"
"strings"
)
func main() {
data, err := ioutil.ReadFile("/etc/passwd")
if err != nil {
log.Fatal(err)
}
lines := strings.Split(string(data), "\n")
for _, line := range lines {
fmt.Println("line:", string(line))
}
}
Having r as an instance of *bufio.Reader, and myList as a slice of strings, than one could just loop and read lines till EOL.
for {
line, err := r.ReadBytes('\n')
if err != nil {
break
}
myList = append(myList, string(line))
}

How to improve this file reading code

I currently have this piece of code that will read a file line by line (delimited by a \n)
file, _ := os.Open(filename) //deal with the error later
defer file.Close()
buf := bufio.NewReader(file)
for line, err := buf.ReadString('\n'); err != io.EOF; line, err = buf.ReadString('\n')
{
fmt.Println(strings.TrimRight(line, "\n"))
}
However I don't feel comfortable with writing buf.ReadString("\n") twice in the for loop, does anyone have any suggestions for improvement?
bufio.ReadString reads until the first occurrence of delim in the input,
returning a string containing the data up to and including the
delimiter. If ReadString encounters an error before finding a
delimiter, it returns the data read before the error and the error
itself (often io.EOF). ReadString returns err != nil if and only if
the returned data does not end in delim.
If buf.ReadString('\n') returns an error other than io.EOF, for example bufio.ErrBufferFull, you will be in an infinite loop. Also, if the file doesn't end in a '\n', you silently ignore the data after the last '\n'.
Here's a more robust solution, which executes buf.ReadString('\n') once.
package main
import (
"bufio"
"fmt"
"io"
"os"
"strings"
)
func main() {
filename := "FileName"
file, err := os.Open(filename)
if err != nil {
fmt.Println(err)
return
}
defer file.Close()
buf := bufio.NewReader(file)
for {
line, err := buf.ReadString('\n')
if err != nil {
if err != io.EOF || len(line) > 0 {
fmt.Println(err)
return
}
break
}
fmt.Println(strings.TrimRight(line, "\n"))
}
}
Most code that reads line by line can be improved by not reading line by line. If your goal is to read the file and access the lines, something like the following is almost always better.
package main
import (
"fmt"
"io/ioutil"
"log"
"strings"
)
func main() {
b, err := ioutil.ReadFile("filename")
if err != nil {
log.Fatal(err)
}
s := string(b) // convert []byte to string
s = strings.TrimRight(s, "\n") // strip \n on last line
ss := strings.Split(s, "\n") // split to []string
for _, s := range ss {
fmt.Println(s)
}
}
Any errors come to you at a single point so error handling is simplified. Stripping a newline off the last line allows for files that may or may not have that final newline, as Peter suggested. Most text files are tiny compared to available memory these days, so reading these in one gulp is appropriate.

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