Each Linux/Unix command has 3 communication channels: input, output and error. The output can be filtered and then redirected and by this way parts of it can be captured, depending on the needs.
Redirecting the output to a file, using > or >>:
> – if the file mentioned does not exist it will be created, and if it does exist it will be overwritten;
ls -la > testfile1.txt
>> – if the file mentioned does not exist it will be created, and if it does exist the information it will be added at the end, without affecting existing data (append);
ls -la >> testfile2.txt
Redirecting the errors to a file, using > or >>:
Identification ID for errors channel is “2“, to capture the error output we must add this digit to our command:
ls -la 2> testfile3.txt
ls -la 2>> testfile4.txt
If we want to remove the output and/or the errors created by a command, we can send them to a special folder /dev/null;
Keep the output, remove the errors:
ls -la 2> /dev/null
Keep the errors, remove the output:
ls -la > /dev/null
Remove output, remove errors:
ls -la > /dev/null 2>&1
Commands, used to redirect the output and the errors, can be combined:
ls -la > testfile5.txt 2> testfile6.txt ls -la >>testfile5.txt 2> testfile6.txt ls -la >>testfile5.txt 2> /dev/null
And examples can continue, as you can see, there are a lot of variations.
If you want to get the output/error on both console and file, you must use the command below:
ls -la 2>&1 | tee testfile7.txt
