Output Redirection, Append, noclobber, and tee

1. > replaces; >> appends

The most common output redirections connect stdout to a regular file. > opens the destination for writing and normally truncates an existing file. >> opens it for append.

printf 'first\n' > report.txt
printf 'second\n' > report.txt
cat report.txt

printf 'third\n' >> report.txt
cat report.txt

2. Prefix the redirection with the descriptor you mean

When no descriptor is written, output redirection defaults to stdout, descriptor 1. To redirect stderr, use descriptor 2 explicitly.

command_that_might_fail() {
  printf 'normal record\n'
  printf 'warning record\n' >&2
}

command_that_might_fail >stdout.log 2>stderr.log

printf '%s\n' '--- stdout.log ---'
cat stdout.log
printf '%s\n' '--- stderr.log ---'
cat stderr.log

3. Merge stderr into the current stdout destination

The form 2>&1 duplicates the destination currently used by stdout onto descriptor 2.

{
  printf 'normal\n'
  printf 'diagnostic\n' >&2
} >combined.log 2>&1

cat combined.log

Bash also supports &>combined.log as a convenient Bash-specific shorthand for redirecting both stdout and stderr to one file. The explicit form is worth learning because it exposes descriptor semantics and is more portable across shell dialects.

4. Redirections are processed from left to right

These two commands are not equivalent:

# Both streams end up in all.log:
some_command >all.log 2>&1

# stderr is duplicated to the shell's OLD stdout first;
# only stdout is then redirected to output.log:
some_command 2>&1 >output.log

flowchart TB
  A["Start: stdout -> terminal; stderr -> terminal"] --> B[" >all.log"]
  B --> C["stdout -> all.log; stderr -> terminal"]
  C --> D[" 2>&1"]
  D --> E["stdout -> all.log; stderr -> all.log"]

5. noclobber can block accidental regular-file truncation

Bash's noclobber option makes a normal > redirection fail when the destination is an existing regular file. It is a useful interactive and scripting guardrail, though it does not replace correct path validation.

set -o noclobber

printf 'protected\n' > protected.txt

# This second write fails instead of truncating the file:
printf 'replacement\n' > protected.txt ||   printf 'overwrite blocked\n' >&2

cat protected.txt

set +o noclobber

When you intentionally need to override noclobber, Bash provides >|. Use that operator only when the overwrite is explicitly part of the design.

set -o noclobber
printf 'forced replacement\n' >| protected.txt
set +o noclobber
cat protected.txt

6. tee copies stdin to files and stdout

tee is useful when you want to preserve a stream in a file while still allowing it to continue through the terminal or pipeline.

printf '%s\n' alpha beta gamma | tee values.log

printf '%s\n' delta | tee -a values.log

cat values.log

flowchart LR
  C["command stdout"] --> T["tee"]
  T --> F["log file"]
  T --> O["tee stdout -> terminal or next pipe"]

Remember that tee receives only the stream connected to its stdin. stderr from the command on the left does not automatically enter the pipe.

7. Include diagnostics in tee only when that is your intention

To send both streams through tee, merge stderr into stdout before the pipe:

{
  printf 'result\n'
  printf 'warning\n' >&2
} 2>&1 | tee combined.log

This is useful for a human-oriented session log. It is usually a bad choice when stdout is a machine-readable API, because merging destroys the distinction between result data and diagnostics.

8. Redirect a whole command group once

Brace groups let several commands share one redirection without opening the file repeatedly.

{
  printf '=== environment ===\n'
  printf 'user=%s\n' "$(whoami)"
  printf 'pwd=%s\n' "$PWD"
  printf 'bash=%s\n' "$BASH_VERSION"
} > environment.txt

cat environment.txt

The closing brace must be followed by a command terminator such as a newline or semicolon. Brace groups run in the current shell context, unlike parenthesized subshell groups.

9. Build logs without corrupting command results

A common production pattern is to keep stdout reserved for a result and write diagnostics to a separate log file. Allocate a descriptor for logging:

log_file="build.log"
exec 3>>"$log_file"

printf 'build started\n' >&3
artifact="/tmp/app.tar.gz"
printf 'artifact=%s\n' "$artifact" >&3

# Clean machine-readable output:
printf '%s\n' "$artifact"

exec 3>&-

This pattern becomes more useful later when functions, timestamps, log levels, and traps are introduced.

10. Redirection does not make multi-command updates atomic

Writing through > can truncate a file before all content is successfully generated. For important configuration files, a safer pattern is often: write a temporary file, validate it, then rename it into place.

target="generated.conf"
tmp=$(mktemp "${target}.XXXXXX")

if generate_configuration >"$tmp"; then
  # validate_configuration "$tmp" would go here
  mv -- "$tmp" "$target"
else
  rm -f -- "$tmp"
  exit 1
fi

11. Hands-on lab: create separate result and session logs

Create a script whose stdout is a single artifact path while stderr is copied both to the terminal and to a persistent log.

mkdir -p "$HOME/devops-academy/bash/chapter03/lesson02"
cd "$HOME/devops-academy/bash/chapter03/lesson02"

cat > build.sh <<'EOF'
#!/usr/bin/env bash

artifact="./output/app.tar.gz"
mkdir -p ./output

printf 'preparing build directory\n' >&2
printf 'fake artifact content\n' > "$artifact"
printf 'build completed\n' >&2

printf '%s\n' "$artifact"
EOF

# Capture stdout as the result.
# Send stderr through tee, but route tee's own stdout back to stderr.
artifact=$(
  bash build.sh 2> >(tee -a build.log >&2)
)

printf 'caller received artifact: %s\n' "$artifact"
printf '%s\n' '--- persistent log ---'
cat build.log

12. Knowledge check

13. Summary

Output redirection is descriptor wiring with real state-changing consequences. Use > only when replacement is intended, >> for append, descriptor numbers to preserve stream separation, and tee when one stream must continue while also being recorded. Redirection order is semantic, not stylistic.

14. Further reading

• GNU Bash Reference Manual — Redirections and the noclobber option.
• GNU Coreutils manual — tee.
• POSIX Shell Command Language — redirection ordering.
• Linux dup(2) manual page — descriptor duplication semantics.