A failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
3.6.1 Redirecting Input
Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is:
[n]<word
3.6.2 Redirecting Output
Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>[|]word
If the redirection operator is ‘>’, and the noclobber option to the set builtin has been enabled, the redirection will fail if the file whose name results from the expansion of word exists and is a regular file. If the redirection operator is ‘>|’, or the redirection operator is
‘>’ and the noclobber option is not enabled, the redirection is attempted even if the file named by word exists.
3.6.3 Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor
1) if n is not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
3.6.4 Redirecting Standard Output and Standard Error
This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.
There are two formats for redirecting standard output and standard error:
&>word and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
When using the second form, word may not expand to a number or ‘-’. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.