Pipes & Redirection

# Output redirection
echo "hello" > file.txt       # Overwrite
echo "world" >> file.txt      # Append

# Input redirection
sort < unsorted.txt
wc -l < /etc/passwd

# Redirect both stdout and stderr
command > file.txt 2>&1        # Traditional
command &> file.txt            # Bash shorthand

# Redirect stderr only
command 2> errors.log

# Discard output
command > /dev/null
command &> /dev/null

# Redirect to /dev/null selectively
command 2>/dev/null            # Discard stderr only
command >/dev/null 2>&1        # Discard everything

# Log both stdout and stderr, but only show stderr
command > output.log 2>&1 | tee /dev/tty

# Separate stdout and stderr into different files
command > stdout.log 2> stderr.log

# Redirect stderr to stdout, then pipe
command 2>&1 | grep "error"

# Only pipe stderr
command 2>&1 >/dev/null | grep "error"

# Append stderr to a log file
command 2>> error.log

# Redirect specific fd
exec 3> custom.log
echo "to custom log" >&3
exec 3>&-

# Split output to file and terminal
echo "hello" | tee output.txt
# Output: hello (displayed)
# Also: written to output.txt

# Append mode
echo "more" | tee -a output.txt

# Pipe and log
command | tee output.log | grep "pattern"

# Write to multiple files
command | tee file1.txt file2.txt file3.txt

# Combine with stderr
command 2>&1 | tee output.log

# Write to a file with sudo (when you don't own the file)
command | sudo tee /etc/config > /dev/null

# <() creates a temp file descriptor with command output
# Compare two command outputs
diff <(ls /dir1) <(ls /dir2)

# Feed a while loop from a pipe
while read -r line; do
    echo "Processing: $line"
done < <(find /var/log -name "*.log" -mtime -1)

# Parallel processing
while read -r host; do
    ssh "$host" uptime &
done < <(cat hosts.txt)
wait

# Combine with here-strings
diff <(sort file1.txt) <(sort file2.txt)

# Two-way process substitution (Bash 4.1+)
coproc myprocess { command; }
echo "input" >&${COPROC[1]}
read -r output <&${COPROC[0]}

# Without pipefail
set +o pipefail
false | true
echo $?    # 0 (last command succeeded)

# With pipefail
set -o pipefail
false | true
echo $?    # 1 (any command in pipe failed)

# Real-world example
command1 | command2 | command3
# Without pipefail: only command3's exit code matters
# With pipefail: the first failed command's exit code is returned

# This is why set -euo pipefail is critical
# It catches failures in pipelines that would otherwise be silent

# Override pipefail for a single pipeline
set +o pipefail
false | true
echo $?    # 0 (pipefail disabled)
set -o pipefail