How to do it...

1. Use the greater-than symbol to append text to a file:

        $ echo "This is a sample text 1" > temp.txt

This stores the echoed text in temp.txt. If temp.txt already exists, the single greater-than sign will delete any previous contents.

1. Use double-greater-than to append text to a file:

        $ echo "This is sample text 2" >> temp.txt

1. Use cat to view the contents of the file:

        $ cat temp.txt
        This is sample text 1
        This is sample text 2

The next recipes demonstrate redirecting stderr. A message is printed to the stderr stream when a command generates an error message. Consider the following example:

$ ls +
ls: cannot access +: No such file or directory

Here + is an invalid argument and hence an error is returned.

The following command prints the stderr text to the screen rather than to a file (and because there is no stdout output, out.txt will be empty):

$ ls + > out.txt
ls: cannot access +: No such file or directory 

In the following command, we redirect stderr to out.txt with 2> (two greater-than):

$ ls + 2> out.txt # works

You can redirect stderr to one file and stdout to another file.

$ cmd 2>stderr.txt 1>stdout.txt

It is also possible to redirect stderr and stdout to a single file by converting stderr to stdout using this preferred method:

$ cmd 2>&1 allOutput.txt

This can be done even using an alternate approach:

$ cmd &> output.txt 

If you don't want to see or save any error messages, you can redirect the stderr output to /dev/null, which removes it completely. For example, consider that we have three files a1, a2, and a3. However, a1 does not have the read-write-execute permission for the user. To print the contents of all files starting with the letter a, we use the cat command. Set up the test files as follows:

$ echo A1 > a1
$ echo A2 > a2
$ echo A3 > a3
$ chmod 000 a1  #Deny all permissions

Displaying the contents of the files using wildcards (a*), will generate an error message for the a1 file because that file does not have the proper read permission:

$ cat a*
cat: a1: Permission denied
A2
A3

Here, cat: a1: Permission denied belongs to the stderr data. We can redirect the stderr data into a file, while sending stdout to the terminal.

$ cat a* 2> err.txt #stderr is redirected to err.txt
A2
A3

$ cat err.txt
cat: a1: Permission denied

Some commands generate output that we want to process and also save for future reference or other processing. The stdout stream is a single stream that we can redirect to a file or pipe to another program. You might think there is no way for us to have our cake and eat it too.

However, there is a way to redirect data to a file, while providing a copy of redirected data as stdin to the next command in a pipe. The tee command reads from stdin and redirects the input data to stdout and one or more files.

command | tee FILE1 FILE2 | otherCommand

In the following code, the stdin data is received by the tee command. It writes a copy of stdout to the out.txt file and sends another copy as stdin for the next command. The cat -n command puts a line number for each line received from stdin and writes it into stdout:

$ cat a* | tee out.txt | cat -n
cat: a1: Permission denied
     1 A2
     2 A3

Use cat to examine the contents of out.txt:

$ cat out.txt
A2
A3

By default, the tee command overwrites the file. Including the -a option will force it to append the new data.

$ cat a* | tee -a out.txt | cat -n

Commands with arguments follow the format: command FILE1 FILE2 ... or simply command FILE.

To send two copies of the input to stdout, use - for the filename argument:

$ cmd1 | cmd2 | cmd -

Consider this example:

$ echo who is this | tee -
who is this
who is this

Alternately, we can use /dev/stdin as the output filename to use stdin.
Similarly, use /dev/stderr for standard error and /dev/stdout for standard output. These are special device files that correspond to stdin, stderr, and stdout.