UNIX for Dummies Questions & Answers

Hi guys,
my doubs are around of this, stdout and sterr, i don't understand, why, when and the tipically situation where it's usseful.
For example:

Code:

#!/bin/bash
# usugrup: datos y grupos de un usuario
#
USUARIO=$1
id $USUARIO 1>/dev/null 2>&1    .....HERE
ERROR=$?
if [ $ERROR -ne 0 ]
then
  echo "El usuario " $USUARIO "no existe"
  exit

if anyone can help me i'd like that explain to me step by step, please.

Moderator's Comments:

Please use code tags next time for your code and data.

The commandid valid_userwrites the user ID and group ID information for valid_user to stout (i.e., file descriptor 1). The 1>/dev/nullthrows away that information. The commandid invalid_userwrites a diagnostic message to stderr (i.e., file descriptor 2) saying that invalid_user is not known. The2>&1 redirects the output written to file descriptor 2 to go to the same place as data written to file descriptor 1. So, for both valid_user and invalid_user, the output from the id command will be thrown away.

When id successfully completes (after writing data for valid_user), it terminates with exit code 0. When id completes unsuccessfully (after writing a diagnostic message for invalid_user), it terminates with a non-zero exit code. Immediately after id completes, the value of$?is the exit status of the id command. TheERROR=$?saves id's exit code in $ERROR.

The following if statement in your script writes a note to stdout (instead of stderr) saying the user doesn't exist if id reported an error and returned a non-zero exit status.

stdout is the file that provides buffered process output for normal data. This means that output going to stdout does not necessarily appear right away.

stderr is unbuffered for error or warning information. Information going here does appear right away.

By convention 0 stdin 1 = stdout 2 = stderr. All processes that have a controlling tty (screen and keyboard: or 0, 1, and 2 redirected to or from some file. When you run a script it becomes stdin, for example.).

If everyone follows the convention then you get errors in one output stream and good stuff separated. >/dev/null 2>&1 means ALL output goes to /dev/null.
The >/dev/null part assigns stdout (1) to the bit bucket, 2>&1 assigns stderr (2) to whatever 1 (stdout) is already "aimed" at. It can be any valid file. /dev/tty is a file and is the controlling terminal.

Sir? the response was great, Now I can understand much better the things, Thanks a lot.