Redirection of standard streams

A POSIX shell can disconnect the standard streams from their default devices and connect them to another devices. This is called redirection of standard streams.

Output redirection

Standard output of a program can be redirected into a file file by the directives >file and >>file. For example, the shell command
```
./hello > out.txt
```
deletes the contents of the file out.txt and then sends the standard output of the program hello into that file.
Alternatively, the shell command
```
./hello >> out.txt
```
appends the file out.txt with whatever hello prints to standard output.
This two redirections are actually an abbreviation of the full commands 1>file and 1>>file where 1 is the default number of the stdout stream.
Analogously the standard error stream of a program can be redirected into a file file by the directives 2>file and 2>>file. For example, the following command redirects standard output into out.txt and standard error into errors.txt.
```
./prog 1> out.txt 2> errors.txt
```
&>file redirects both stdout and stdin into file.
2>&1 redirects stderr to stdout.

Input redirection and pipes

The standard input stream of a program can be attached to a file with the directive <, for example the shell command
```
./prog < input.txt
```
connects the standard input stream of the program prog with the file file such that all reads from the standard input actually read from the attached file.
The pipe, |, connects the standard output of one program to the standard input of another program, for example,
```
echo 1.23 | ./prog
```
sends the string "1.23" into the standard input of the program ./prog.