Pipes & Redirection

#!/bin/bash

# Redirection
command > file.txt     # redirect stdout to file (overwrite)
command >> file.txt    # redirect stdout to file (append)
command 2> error.txt   # redirect stderr to file
command 2>&1           # redirect stderr to stdout
command > all.txt 2>&1 # redirect both to file
command < input.txt    # redirect file to stdin
command &> all.txt     # bash shorthand for both stdout+stderr

# /dev/null — the bit bucket
command > /dev/null         # discard stdout
command > /dev/null 2>&1    # discard everything

# Pipes — connect stdout of one command to stdin of next
ls -la | grep ".sh"            # filter ls output
cat /etc/passwd | cut -d: -f1 | sort    # get and sort usernames
ps aux | grep nginx | grep -v grep      # find nginx processes
du -sh */ | sort -h                      # show dir sizes, sorted
history | awk '{print $2}' | sort | uniq -c | sort -rn | head -10  # top commands

# tee — write to file AND pass through to stdout
command | tee output.txt         # see output AND save it
command | tee -a output.txt      # append instead of overwrite

# xargs — build commands from stdin
find . -name "*.log" | xargs rm          # delete all .log files
find . -name "*.py" | xargs wc -l       # count lines in each file
echo "apple banana cherry" | xargs -n1  # one item per line

# Process substitution (bash-specific)
diff <(ls dir1/) <(ls dir2/)    # compare output of two commands