stdin, stdout, stderr, and File Descriptors

1. Every process starts with an I/O contract

On Unix-like systems, a newly started process normally receives three already-open file descriptors. Bash and the programs it launches use these descriptors as conventional channels for input and output.

A file descriptor is a small integer that a process uses to refer to an open I/O resource. The resource does not have to be a regular file. It can represent a terminal, pipe, socket, device, or other kernel-managed object.

flowchart LR
  T["Terminal keyboard"] -->|"fd 0 / stdin"| P["Process"]
  P -->|"fd 1 / stdout"| O["Terminal display"]
  P -->|"fd 2 / stderr"| E["Terminal display"]

2. stdout and stderr exist for different audiences

Normal output is often intended for another program. Diagnostics are intended to explain what the program is doing or why it failed. Keeping them separate lets automation consume clean data while operators still receive useful errors.

# stdout carries the requested data.
printf 'artifact-2026.08.09.tar.gz\n'

# stderr carries a diagnostic.
printf 'warning: cache was unavailable\n' >&2

If a script emits JSON on stdout, logging warnings into the same stream can make the JSON invalid. A production-friendly command therefore treats stdout as an interface, not merely as a place to print text.

3. Bash redirection syntax operates on file descriptors

The expression >&2 means “send this command's standard output to the destination currently referenced by descriptor 2.” The left side defaults to descriptor 1 when omitted.

printf 'normal output\n'
printf 'diagnostic output\n' >&2

# The long form is equivalent for stdout:
printf 'diagnostic output\n' 1>&2

Redirection is processed by the shell before the target command executes. The program usually does not need to know whether its stdout points at a terminal, file, or pipe.

4. Child processes inherit open descriptors

When Bash launches a child process, the child normally inherits Bash's open standard descriptors unless redirections replace them. This is why one shell-level redirection can affect an entire command invocation.

flowchart TB
  B["Bash"] -->|"launch child"| C["Child process"]
  B -. "fd 0" .-> I["input source"]
  B -. "fd 1" .-> O["output destination"]
  B -. "fd 2" .-> E["diagnostic destination"]
  C -. "inherits / redirected" .-> I
  C -. "inherits / redirected" .-> O
  C -. "inherits / redirected" .-> E

5. Exit status is not another output stream

A command's exit status is a small integer reported to its parent when the command terminates. It is not printed on stdout or stderr unless some program explicitly prints it.

grep 'needle' /etc/hosts
status=$?

printf 'grep exit status = %d\n' "$status" 

This separation is fundamental:

• stdout can contain a result.
• stderr can contain diagnostics.
• exit status tells the caller whether the command considered the operation successful.

A command can produce no output and still succeed. It can also print useful output and later fail.

6. Programs can behave differently when attached to a terminal

The test -t FD condition checks whether a file descriptor refers to a terminal. Many CLI programs use terminal detection to decide whether to enable colors, progress bars, prompts, or interactive formatting.

for fd in 0 1 2; do
  if [[ -t $fd ]]; then
    printf 'fd %d is a terminal\n' "$fd"
  else
    printf 'fd %d is not a terminal\n' "$fd"
  fi
done

Try the command normally, then redirect its stdout to a file. Descriptor 1 will stop being a terminal. This explains why some commands automatically change formatting inside CI logs or pipelines.

7. Linux exposes descriptors through /proc

On Linux, a process's open descriptors can be inspected through /proc/PID/fd. For the current Bash process, $$ is its process ID.

ls -l "/proc/$$/fd" 2>/dev/null ||   printf '/proc descriptor inspection is not available here\n'

You may see symbolic links for descriptors 0, 1, and 2 pointing to a terminal device, pipe, or other resource. This inspection is Linux-specific; the underlying descriptor concept is broader than Linux.

8. Design scripts with a clean stream contract

Suppose a script must return one artifact path for a downstream job while also describing progress to humans. Put the path on stdout and progress on stderr:

#!/usr/bin/env bash

artifact="/tmp/build/app.tar.gz"

printf 'building artifact...\n' >&2
# build steps would run here
printf 'build complete\n' >&2

# Machine-readable result:
printf '%s\n' "$artifact" 

A caller can then capture only the result:

artifact=$(bash build.sh)
printf 'deploying %s\n' "$artifact" 

9. Bash can use descriptors beyond 0, 1, and 2

Scripts sometimes allocate additional descriptors for logs, lock files, or separate channels. Modern Bash can choose an unused descriptor automatically.

log_file=$(mktemp)

exec {logfd}>"$log_file"
printf 'allocated descriptor: %s\n' "$logfd"
printf 'structured log line\n' >&"$logfd"
exec {logfd}>&-

cat "$log_file"
rm -f "$log_file" 

The exec builtin changes the current shell's descriptor table when it is used with redirections and no external command. This is powerful, but keep descriptor lifetimes explicit and close resources you no longer need.

10. Hands-on lab: prove stream separation

Create a script that intentionally writes one record to stdout and three diagnostics to stderr. Capture the streams separately and verify the exit status independently.

mkdir -p "$HOME/devops-academy/bash/chapter03/lesson01"
cd "$HOME/devops-academy/bash/chapter03/lesson01"

cat > stream-demo.sh <<'EOF'
#!/usr/bin/env bash

printf 'starting lookup\n' >&2
printf 'checking local cache\n' >&2
printf '{"service":"api","status":"ready"}\n'
printf 'lookup complete\n' >&2
exit 0
EOF

bash stream-demo.sh >result.json 2>diagnostics.log
status=$?

printf 'exit=%d\n' "$status"
printf '%s\n' '--- stdout ---'
cat result.json
printf '%s\n' '--- stderr ---'
cat diagnostics.log

11. Knowledge check

12. Summary

Standard streams are the foundation of composable Unix automation. Descriptor 0 is stdin, 1 is stdout, and 2 is stderr. Bash can reconnect those descriptors before launching commands, allowing files, terminals, pipes, and processes to be combined without modifying the programs themselves. Preserve the distinction between normal output, diagnostics, and exit status.

13. Further reading

• GNU Bash Reference Manual — Redirections.
• POSIX Shell Command Language — redirection and command execution.
• Linux open(2), dup2(2), and proc(5) manual pages.
• GNU Coreutils documentation — standard input and output conventions.