Redirections

command < input_file

# Count lines in a file
wc -l < file.txt

# Sort file contents
sort < unsorted.txt

# Read and display file
cat < document.txt

# Multiple commands with input redirection
grep "error" < logfile.txt | wc -l

# File doesn't exist
cat < nonexistent.txt
# Output: nonexistent.txt : No such file or directory
# Exit status: 1

# Permission denied
cat < /root/secret.txt
# Output: /root/secret.txt : Permission denied
# Exit status: 1

command > output_file

# Save command output
ls -la > directory_listing.txt

# Create or overwrite file
echo "Hello World" > greeting.txt

# Save error output (stderr not redirected in basic >)
ls /nonexistent > errors.txt
# errors.txt will be empty; error appears on screen

# Empty a file
> empty_file.txt
echo "" > empty_file.txt

# Creates file with permissions 0666 (rw-rw-rw-)
echo "content" > newfile.txt

# Overwrites existing file
echo "first" > file.txt
echo "second" > file.txt
cat file.txt
# Output: second

command >> output_file

# Append to log file
echo "[INFO] Application started" >> app.log
echo "[INFO] Loading configuration" >> app.log

# Build file incrementally
echo "First line" >> document.txt
echo "Second line" >> document.txt
echo "Third line" >> document.txt

cat document.txt
# Output:
# First line
# Second line
# Third line

# Append command output
ls -la >> inventory.txt
date >> inventory.txt

# Creates file if it doesn't exist
echo "New content" >> newfile.txt

# Appends if file exists
echo "More content" >> newfile.txt

command << DELIMITER
content line 1
content line 2
DELIMITER

# Basic heredoc
cat << EOF
This is line 1
This is line 2
This is line 3
EOF
# Output:
# This is line 1
# This is line 2
# This is line 3

# Common delimiters (any word works)
cat << END
Content here
END

cat << STOP
More content
STOP

# Heredoc with grep
grep "error" << EOF
info: starting
error: failed to connect
warning: retrying
error: timeout
EOF
# Output:
# error: failed to connect
# error: timeout

# Create multi-line file
cat << EOF > config.txt
server=localhost
port=8080
timeout=30
EOF

# Heredoc with variable expansion
cat << EOF
User: $USER
Home: $HOME
EOF
# Output:
# User: john
# Home: /home/john

cat << END
> First line
> Second line
> Third line
> END

# Last redirection takes precedence
cat < file1.txt < file2.txt < file3.txt
# Reads from file3.txt only

# Last redirection takes precedence
echo "Hello" > file1.txt > file2.txt
# Writes to file2.txt only

# Read from input.txt, write to output.txt
sort < input.txt > output.txt

# Same with append
grep "error" < logfile.txt >> errors.txt

# Heredoc to file
cat << EOF > config.txt
key=value
another=setting
EOF

# These are equivalent
command < input.txt > output.txt
command > output.txt < input.txt

# Redirections can be anywhere
< input.txt command > output.txt

# Output redirection after pipe
cat file.txt | grep "error" > errors.txt

# Input redirection before pipe
< input.txt sort | uniq > output.txt

# Multiple pipes and redirections
< data.txt grep "keyword" | sort | uniq > results.txt

# Heredoc with pipe
cat << EOF | grep "error"
info: started
error: failed
warning: slow
EOF

# Input file doesn't exist
cat < missing.txt
# Output: missing.txt : No such file or directory
# Exit status: 1

# Cannot create output file
echo "test" > /root/file.txt
# Output:  : No such file or directory
# Exit status: 1

# Missing filename
cat < 
# Output: syntax error: redir

# Multiple redirections without file
cat < < file.txt
# Output: syntax error: redir

# Invalid combinations
cat <> file.txt
# Output: syntax error: redir

# Complex pipeline with redirections
< input.txt grep "error" | sort >> sorted_errors.txt

# Heredoc with multiple commands
cat << EOF | grep "user" | sort > users.txt
user: john
admin: root
user: jane
guest: visitor
EOF

# Process file and save results
sort < unsorted.txt > sorted.txt
uniq < sorted.txt > unique.txt
wc -l < unique.txt > count.txt

# Build log file over time
date >> application.log
echo "Starting service" >> application.log
ls -la >> application.log
echo "Service started" >> application.log