Bash Redirections: Complete Linux Cheat Sheet

In Bash, redirection controls where a command reads input from and where it sends output to. Mastering redirections is essential for writing robust scripts, saving logs, handling errors, and chaining commands. This cheat-sheet explains the common redirection operators, file-descriptor tricks, here-documents, process substitution, and practical examples you can use immediately.

Basic Output Redirections ​

• cmd > file
  Redirect standard output (stdout) of cmd to file (overwrite).
• cmd 1> file
  Same as cmd > file - 1 is the file descriptor (fd) for stdout.
• cmd >> file
  Append stdout of cmd to file.

Example:

echo "Hello" > out.txt
echo "More" >> out.txt

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Error Redirections (stderr) ​

• cmd 2> file
  Redirect standard error (stderr) to file.
• cmd 2>> file
  Append stderr to file.

Example:

ls /no/such/path 2> errors.log

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Redirecting Both stdout and stderr ​

• cmd &> file
  Redirect both stdout and stderr to file (bash shorthand).
• cmd > file 2>&1
  Redirect stdout to file then redirect stderr to where stdout is pointing. Order matters.

Correct usage:

command > all.log 2>&1
# or (bash-only)
command &> all.log

Warning: cmd 2>&1 > file is not equivalent; that order redirects stderr to current stdout (typically the terminal) before stdout is redirected to file.

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Discard Output (silence) ​

• cmd > /dev/null - discard stdout
• cmd 2> /dev/null - discard stderr
• cmd &> /dev/null - discard both

Example:

grep pattern bigfile > /dev/null 2>&1

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Input Redirections ​

• cmd < file - Use file as stdin for cmd.

Here-documents (multi-line input):

cat <<EOF
line1
line2
EOF

If EOF is quoted (<<'EOF'), the content is literal (no variable expansion). Use <<-EOF to strip leading tabs.

Here-strings (single line):

grep foo <<< "single-line input"

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Process Substitution (anonymous FIFOs) ​

Process substitution lets you treat command output like a file.

• cmd <(cmd1) - pass stdout of cmd1 as a filename argument to cmd.
• cmd >(cmd2) - create a pipe where cmd writes and cmd2 reads.

Example (compare two directories):

diff <(ls /path1 | sort) <(ls /path2 | sort)

Example (send different streams to different handlers):

command > >(grep "info" > info.log) 2> >(grep "error" > error.log)

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Pipes and Combined Streams | ​

• cmd1 | cmd2 - pipe stdout of cmd1 to stdin of cmd2.
• cmd1 |& cmd2 - pipe stdout and stderr (bash 4+). For older bash: cmd1 2>&1 | cmd2.

Get exit codes of all piped commands:

cmd1 | cmd2 | cmd3
echo ${PIPESTATUS[@]}

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Advanced File Descriptor Tricks ​

Bash supports custom file descriptors and exec to manipulate them:

• exec 3> file - open file for writing on fd 3.
• exec 3< file - open file for reading on fd 3.
• exec 3>&- - close fd 3.
• echo "text" >&3 - write to fd 3.
• cat <&3 - read from fd 3.

Copying and closing fds:

• exec 4>&3 - duplicate fd 3 to fd 4.
• exec 4>&3- - copy fd 3 to 4 and close 3.

Example:

exec 3> logfile.txt
echo "Start" >&3
exec 3>&-

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Redirecting Multiple Commands (groups) ​

• Subshell grouping ( ... ) runs in a subshell:

( cmd1; cmd2 ) > out.txt

• Compound grouping { ...; } runs in the current shell (no subshell):

{ cmd1; cmd2; } > out.txt

Use {} when you want the redirection to affect the current shell and to avoid subshell overhead.

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Network I/O via /dev/tcp and /dev/udp (bash feature) ​

Bash can open TCP/UDP connections using exec on special device files (not available on all shells):

exec 3<> /dev/tcp/example.com/80
printf 'GET / HTTP/1.0\r\nHost: example.com\r\n\r\n' >&3
cat <&3
exec 3>&-

This is a lightweight way to perform basic socket operations in bash scripts.

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Practical Examples & Tips ​

1. Log both output and errors and still see them on screen:

command 2>&1 | tee -a combined.log

1. Swap stdout and stderr for a command:

command 3>&1 1>&2 2>&3

1. Run a long job, background it, and redirect output:

longjob > job.out 2> job.err &

1. Debugging scripts:

set -x   # enable debug trace
# run commands...
set +x   # disable debug trace

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Common Pitfalls to Avoid ​

• Pay attention to the order when mixing [ICODE]2>&1[/ICODE] and > - wrong order leads to unexpected results.
• Remember that &> is bash-specific (not POSIX). For maximum portability use > file 2>&1.
• Don’t forget to close custom file descriptors (exec 3>&-) to avoid resource leaks in long-running scripts.
• Quoting matters in here-documents: unquoted delimiter prevents variable expansion.

Conclusion - Control Your I/O Like a Pro ​

Redirections are powerful and let you shape how programs interact with files, devices, and each other. Once you practice the basics - stdout, stderr, pipes, here-docs, and file descriptors - you’ll be able to build concise, reliable scripts and handle complex workflows without external tools. Keep this cheat-sheet handy and try the examples in a test shell.