Standard Input, Output, Error, and Exit Status

1. Commands expose data streams and a status result

A process normally begins with three open file descriptors. They may point to a terminal, regular file, pipe, socket, or another endpoint. The shell configures these descriptors before the program starts.

flowchart LR
  I["fd 0: standard input"] --> P["Command process"]
  P --> O["fd 1: standard output"]
  P --> E["fd 2: standard error"]
  P --> S["Exit status 0 to 255"]
  O --> N["Terminal, file, or pipe"]
  E --> D["Terminal, file, or log"]

2. Observe stdout, stderr, and status separately

demo_streams() {
  printf 'normal-result\n'
  printf 'diagnostic-message\n' >&2
  return 7
}

demo_streams
status=$?
printf 'captured status=%d\n' "$status"

The function writes one line to descriptor 1, another to descriptor 2, and returns status 7. On a terminal both lines may look similar, but redirection can separate them. Always capture $? immediately; another command replaces it.

3. Redirect input and output

lab="$HOME/devops-academy/linux/chapter03/lesson05/redirection"
mkdir -p -- "$lab"

printf 'first\n' > "$lab/output.txt"
printf 'second\n' >> "$lab/output.txt"

# Separate normal output and diagnostics.
{
  printf 'result\n'
  printf 'warning\n' >&2
} > "$lab/stdout.log" 2> "$lab/stderr.log"

# Feed a file to standard input.
wc -l < "$lab/output.txt"

4. Redirections are processed from left to right

# Both stdout and stderr end in combined.log.
command >combined.log 2>&1

# stderr is duplicated to the original stdout first; only stdout then moves.
# The two streams are NOT combined in combined.log.
command 2>&1 >combined.log

Descriptor duplication copies a destination at that point in the redirection sequence. This distinction is critical in CI logs, service wrappers, and incident capture.

5. Pipelines connect stdout to stdin

printf '%s\n' worker api worker scheduler api \
  | sort \
  | uniq -c \
  | sort -nr

The shell creates pipes and starts the component commands. Standard output from one command becomes standard input to the next. Standard error is not piped unless redirected explicitly.

6. Exit status is the automation contract

By convention, status 0 means success and a nonzero value indicates some kind of failure or alternate condition defined by the command. Shell built-ins, external programs, functions, and pipelines all produce a status.

if grep -q -- '^enabled=true$' app.env; then
  printf 'feature enabled\n'
else
  status=$?
  case "$status" in
    1) printf 'feature not enabled\n' ;;
    *) printf 'grep failed with status %d\n' "$status" >&2; exit "$status" ;;
  esac
fi

mkdir -p build && printf 'build directory ready\n'
test -f required.conf || printf 'required.conf is missing\n' >&2

Some commands use distinct nonzero statuses for “no match” versus execution error. Do not collapse every nonzero value into the same meaning without consulting documentation.

7. Preserve pipeline failure information

set -o pipefail

if output=$(generate_report | compress_report | upload_report); then
  printf 'pipeline completed\n'
else
  status=$?
  printf 'pipeline failed with status %d\n' "$status" >&2
  printf 'component statuses: %s\n' "${PIPESTATUS[*]}" >&2
  exit "$status"
fi

pipefail makes a pipeline fail when a component fails, using Bash’s documented rule for the resulting status. PIPESTATUS records component statuses, but it must be captured before another command overwrites it. Reliable shell automation will be developed fully in Chapters 14 and 15.

8. Capture results and diagnostics as evidence

run_and_capture() {
  if [ "$#" -lt 2 ]; then
    printf 'usage: run_and_capture NAME COMMAND [ARG...]\n' >&2
    return 2
  fi

  name=$1
  shift
  evidence="$HOME/devops-academy/linux/chapter03/lesson05/evidence"
  mkdir -p -- "$evidence"

  stdout_file="$evidence/$name.stdout.log"
  stderr_file="$evidence/$name.stderr.log"
  status_file="$evidence/$name.status"

  printf 'command:' > "$evidence/$name.command"
  printf ' %q' "$@" >> "$evidence/$name.command"
  printf '\n' >> "$evidence/$name.command"

  "$@" > "$stdout_file" 2> "$stderr_file"
  status=$?
  printf '%d\n' "$status" > "$status_file"
  return "$status"
}

run_and_capture kernel uname -a
printf 'kernel status=%d\n' "$?"

run_and_capture missing ls -ld -- /path/that/does/not/exist || true
cat "$HOME/devops-academy/linux/chapter03/lesson05/evidence/missing.stderr.log"

The %q representation is useful for shell-oriented evidence, but it is not a cryptographic audit record and must not be used to log secrets.

9. Hands-on lab: build a stream-aware diagnostic report

lab="$HOME/devops-academy/linux/chapter03/lesson05"
evidence="$lab/lab-evidence"
mkdir -p -- "$evidence"

check_target() {
  target=$1
  printf 'checking=%s\n' "$target"
  if [ -e "$target" ]; then
    stat --printf='type=%F size=%s owner=%U group=%G\n' -- "$target"
    return 0
  fi
  printf 'target does not exist: %s\n' "$target" >&2
  return 4
}

for target in /etc/os-release /path/that/does/not/exist; do
  safe_name=$(printf '%s' "$target" | tr '/ ' '__')
  stdout_file="$evidence/${safe_name}.out"
  stderr_file="$evidence/${safe_name}.err"
  status_file="$evidence/${safe_name}.status"

  if check_target "$target" > "$stdout_file" 2> "$stderr_file"; then
    status=0
  else
    status=$?
  fi
  printf '%d\n' "$status" > "$status_file"
done

printf '=== evidence inventory ===\n'
find "$evidence" -maxdepth 1 -type f -printf '%f\n' | sort

printf '\n=== missing-target evidence ===\n'
cat "$evidence/_path_that_does_not_exist.err"
printf 'status='
cat "$evidence/_path_that_does_not_exist.status"

10. Common stream and status mistakes

11. Knowledge check

12. Chapter summary

Chapter 3 established the command-line execution environment: terminal and shell layers, working-directory navigation, pathname resolution and expansion, interactive productivity features, standard streams, and exit status. These concepts make later file, process, service, network, and automation commands observable and composable.

13. Further reading

• GNU Bash Reference Manual — redirections, pipelines, lists, exit status, functions, and shell options.
• Linux manual pages for stdin, stdout, stderr, dup2, pipe, bash, printf, and test.
• POSIX Shell Command Language — redirection, pipelines, command lists, and exit status.
• GNU Coreutils manuals — conventions for standard streams and command status.
• systemd and CI runner documentation — stdout/stderr capture and process exit-code handling.