diff is not working in execSync producing error when file doesnot match

Question

diff command gives error when my files does not match const exec_options = { cwd : "/home" , encoding : 'utf8', }

    let {stdout,stderr,err} =    execSync(`diff  output.txt answer.txt` , exec_options);
if(err){
    console.log(err);
}
console.log(stdout);

The above command gives desired result with exec and in terminal but does not work using exec sync

Answer 1

execSync returns string or buffer

Your code should be..

const stdout = execSync(`diff output.txt answer.txt` , exec_options);
console.log(stdout.toString());

if you want to configure stderr and stdout, uses spawnSync

Why execSync is fail ?

https://askubuntu.com/questions/698784/exit-code-of-diff

because, diff returns non-zero when something is different.

and execSync determines normal termination only when the value is 0.

so if something is different, diff returns 1, and execSync guess command failed.

actually we get that message.

  try {
    let stdout = execSync(`diff a b`, exec_options)
    console.log(stdout.toString())
  } catch (e: any) {
    console.log(e.message)
    console.log(e.stdout.toString())
  }

Command failed: diff a b
2c2
< a
---
> b

some test

  try {
    const command = 'diff a c'
    console.log(`command : ${command}`)
    let stdout = execSync(command, exec_options)
    console.log(stdout.toString())
  } catch (e: any) {
    console.log('prev')
    if (e.status === 1) {
      console.log('diff result:')
      console.log(e.stdout.toString())
      console.log(e.stderr.toString())
    } else {
      console.log('error message : ')
      console.log(e.stdout.toString())
      console.log(e.stderr.toString())
    }
  }

will print

command : diff a a // in same case
                   // nothing returns
~~~~~~~~
command : diff a b // in diff case
prev
diff result:
2c2                // e.stdout exist
< a
---
> b
                   // e.stderr doesn't exist
~~~~~~~~
command : diff a c // in fail case
diff: c: No such file or directory // print at execSync.
prev
error message : 
                   // e.stdout doesn't exist
diff: c: No such file or directory // e.stderr exist