Redirecting or saving output text to a file can be done as follows:

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

This would store the echoed text in temp.txt by truncating the file, the contents will be emptied before writing.

To append text to a file, consider the following example:

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

You can view the contents of the file as follows:

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

Let us see what a standard error is and how you can redirect it. stderr messages are printed when commands output 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:

$ ls + 2> out.txt # works

You can redirect stderr exclusively to a file and stdout to another file as follows:

$ 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 output.txt

Or the alternate approach:

$ cmd &> output.txt 

Sometimes, the output may contain unnecessary information (such as debug messages). If you don't want the output terminal burdened with the stderr details then you should 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. When you need to print the contents of files starting with a, we use the cat command. Set up the test files as follows:

$ echo a1 > a1 
$ cp a1 a2 ; cp a2 a3;
$ chmod 000 a1  #Deny all permissions

While displaying contents of the files using wildcards (a*), it will show an error message for file a1 as it does not have the proper read permission:

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

Here, cat: a1: Permission denied belongs to the stderr data. We can redirect the stderr data into a file, whereas stdout remains printed in the terminal. Consider the following code:

$ cat a* 2> err.txt #stderr is redirected to err.txt
a1
a1

$ cat err.txt
cat: a1: Permission denied

Take a look at the following code:

$ cmd 2>/dev/null

When redirection is performed for stderr or stdout, the redirected text flows into a file. As the text has already been redirected and has gone into the file, no text remains to flow to the next command through pipe (|), and it appears to the next set of command sequences through stdin.

However, there is a way to redirect data to a file, as well as provide a copy of redirected data as stdin for the next set of commands. This can be done using the tee command. For example, to print stdout in the terminal as well as redirect stdout into a file, the syntax for tee is as follows:

command | tee FILE1 FILE2

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
     1a1
     2a1

Examine the contents of out.txt as follows:

$ cat out.txt
a1
a1

Note that cat: a1: Permission denied does not appear because it belongs to stderr. The tee command can read from stdin only.

By default, the tee command overwrites the file, but it can be used with appended options by providing the -a option, for example, $ cat a* | tee -a out.txt | cat -n.

Commands appear with arguments in the format: command FILE1 FILE2 … or simply command FILE.

We can use stdin as a command argument. It can be done by using - as the filename argument for the command as follows:

$ cmd1 | cmd2 | cmd -

For 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.