How to execute system command with unknown arguments? - go

Question

I have a bunch of systems commands which are somwhat similar to appending new content to a file. I wrote a simple script to execute system commands, which works well if there are single words like 'ls' , 'date' etc. But if the command is greater than that, program dies.
The following is the code
package main
import (
"fmt"
"os/exec"
"sync"
)
func exe_cmd(cmd string, wg *sync.WaitGroup) {
fmt.Println(cmd)
c = cmd.Str
out, err := exec.Command(cmd).Output()
if err != nil {
fmt.Println("error occured")
fmt.Printf("%s", err)
}
fmt.Printf("%s", out)
wg.Done()
}
func main() {
wg := new(sync.WaitGroup)
wg.Add(3)
x := []string{"echo newline >> foo.o", "echo newline >> f1.o", "echo newline >> f2.o"}
go exe_cmd(x[0], wg)
go exe_cmd(x[1], wg)
go exe_cmd(x[2], wg)
wg.Wait()
}
The following is the error i see
exec: "echo newline >> foo.o": executable file not found in $PATHexec:
"echo newline >> f2.o": executable file not found in $PATHexec:
"echo newline >> f1.o": executable file not found in $PATH
I guess, this may be due to, not sending cmds and arguments seperately ( http://golang.org/pkg/os/exec/#Command ). I am wondering how to subvert this, since I don't know how many arguments will be there in my command which needs to be executed.

Answer 1

I found a relatively decent way to achieve the same .
out, err := exec.Command("sh","-c",cmd).Output()
Works for me until now. Still finding better ways to achieve the same.
Edit1:
Finally a easier and efficient (atleast so far) way to do would be like this
func exeCmd(cmd string, wg *sync.WaitGroup) {
fmt.Println("command is ",cmd)
// splitting head => g++ parts => rest of the command
parts := strings.Fields(cmd)
head := parts[0]
parts = parts[1:len(parts)]
out, err := exec.Command(head,parts...).Output()
if err != nil {
fmt.Printf("%s", err)
}
fmt.Printf("%s", out)
wg.Done() // Need to signal to waitgroup that this goroutine is done
}
Thanks to variadic arguments in go and people that pointed that out to me :)

Answer 2

For exec.Command() the first argument needs to be the path to the executable. Then the remaining arguments will be supplied as arguments to the executable. Use strings.Fields() to help split the word into a []string.
Example:
package main
import (
"fmt"
"os/exec"
"sync"
"strings"
)
func exe_cmd(cmd string, wg *sync.WaitGroup) {
fmt.Println(cmd)
parts := strings.Fields(cmd)
out, err := exec.Command(parts[0],parts[1]).Output()
if err != nil {
fmt.Println("error occured")
fmt.Printf("%s", err)
}
fmt.Printf("%s", out)
wg.Done()
}
func main() {
wg := new(sync.WaitGroup)
commands := []string{"echo newline >> foo.o", "echo newline >> f1.o", "echo newline >> f2.o"}
for _, str := range commands {
wg.Add(1)
go exe_cmd(str, wg)
}
wg.Wait()
}
Here's an alternative approach that just writes all the commands to a file then executes that file within the context of the new created output directory.
Example 2
package main
import (
"os"
"os/exec"
"fmt"
"strings"
"path/filepath"
)
var (
output_path = filepath.Join("./output")
bash_script = filepath.Join( "_script.sh" )
)
func checkError( e error){
if e != nil {
panic(e)
}
}
func exe_cmd(cmds []string) {
os.RemoveAll(output_path)
err := os.MkdirAll( output_path, os.ModePerm|os.ModeDir )
checkError(err)
file, err := os.Create( filepath.Join(output_path, bash_script))
checkError(err)
defer file.Close()
file.WriteString("#!/bin/sh\n")
file.WriteString( strings.Join(cmds, "\n"))
err = os.Chdir(output_path)
checkError(err)
out, err := exec.Command("sh", bash_script).Output()
checkError(err)
fmt.Println(string(out))
}
func main() {
commands := []string{
"echo newline >> foo.o",
"echo newline >> f1.o",
"echo newline >> f2.o",
}
exe_cmd(commands)
}

Answer 3

out, _ := exec.Command("sh", "-c", "date +\"%Y-%m-%d %H:%M:%S %Z\"").Output()
exec.Command("sh","-c","ls -al -t | grep go >>test.txt").Output()
fmt.Printf("%s\n\n",out)
Tested couple cases and all work good. This is a lifesaver if you are dealing with quick shell commands in your program. Not tested with complex cases.