Input/Output Redirection

📌 Bash Redirection

• In Bash (or other Linux shells), when a program is executed, it uses three standard Input/Output streams, each one represented by a numeric file descriptor:

  • 0 - stdin : the standard input stream (printed on the screen by default)

  • 1 - stdout : the standard output stream

  • 2 - stderr : the standard error stream (printed on the screen by default)

• All three streams can be redirected.

sudo ls /root 
	[sudo] password for user: # This is the std input
	snap	# This is the std output

ls /root
	ls: cannot open directory '/root': Permission denied # This is the std error

Redirection symbols

• The > symbol is used for redirect stdout to a file.

  • If the redirection points to a file that already exists, this file will be overwritten!

ls /etc/ > etc-contents.txt
# The content of the file is the same as the normal content of the command

• The >> symbol is used to append stdout to a file.

ls /tmp/ >> etc-contents.txt

• The < symbol is used for redirect stdin input to a command.

head < /etc/passwd
# The shell took the content of the passwd file and sent it to the head utility

• Redirect standard error to a file.

  • To suppress the error messagges from being displayed on the screen, redirect stderr to /dev/null.

find / -name 'file1.txt' 2> errors.txt
    /home/user/Documents/file1.txt
    /home/user/Documents/dir1/file1.txt

ls -lah errors.txt 
	-rw-rw-r-- 1 user user 89K ott 10 21:07 errors.txt

# Redirect to /dev/null
find / -name 'file1.txt' 2> /dev/null

# Redirect both stdout and stderr to the same file
find / -name 'file1.txt' &> all.txt

# Redirect stdout to all.txt and stderr to the same location as stdout
find / -name 'file1.txt' > all.txt 2>&1

Pipes

• | - Pipes can connect the stdout of one command to the stdin of another command.

ls -l /etc/ | less
# Send the output of the ls command as input to the less command

ls -l /etc/ | head -n 20 | tail -n 5

find / -name 'file1.txt' | less

find / -name 'file1.txt' |& less
# Send stdout and stderr to less command

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