stdin: < : code 0 : Standard input
stdout: > : code 1 : Standard output
stderr: 2> : code 2 : Standard error
| : redirects the output of a command to another command
>> : append to a file
|| : logical OR (if the 1st command fails, do the 2nd anyway)
&& : logical AND (if the 1st command fails, stop)

Redirect stdout to a file (stderr will be displayed on screen and ):

# find /var/log -perm 0755 > /root/perms.txt

Redirect stderr to a file (stdout will be displayed on screen):

# find /var/log -perm 0755 2> /root/perms.txt

Redirect stderr to the target of stdout (here, a file called /root/perms.txt):

# find /var/log -perm 0755 > /root/perms.txt 2>&1

Redirect the echo command to a file:

# echo "test" > /root/testfile

Append text to a file:

# echo " and 2nd test" >> /root/testfile

Redirect to stdin (here, the screen):

# < testfile

Complex command to send a string and attach a file to an mail, and confirm if the command successfully exited:

# echo "Hi" | mailx -s "Re: requested file" root < testfile | if [ $? -eq 0 ]; then; echo "Mail has been sent"; else; echo "Error"; fi


Usage for || and &&:

Example for logical OR:

rm -Rf somedir || exit_on_error "Failed to remove the directory"

Example for logical AND (excludes 2nd part if the first fails):

rm -Rf somedir && trace_output "Removed the directory"

Example for both logical AND and OR:

mailx -s "Re: requested file" root < testfile && echo "Mail has been sent" || echo "Error"

rm -Rf somedir && trace_output "Removed the directory" || exit_on_error "Failed to remove the directory"