📌 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
- 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 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