Redirection

Each pipe symbol in the previous pipeline example instructs the shell to feed output from one command into the input of another. This action is a special form of redirection, which allows you to manage the origin of input streams and the destination of output streams. In the previous example, individual programs are unaware that their output is being handed off to or from another program because the shell takes care of the redirection on their behalf.

Redirection can also occur to and from files. For example, rather than sending the output of an inode list to the pager less, it could easily be sent directly to a file with the > redirection operator:

$ ls -i * | awk '{print $1}' | sort -nu > in.txt

When you change the last redirection operator, the shell creates an empty file (in.txt) and opens it for writing, and the standard output of sort places the results in the file instead of on the screen. Note that, in this example, anything sent to standard error is still displayed on the screen. In addition, if your specified file, in.txt, already existed in your current directory, it would be overwritten.

Since the > redirection operator creates files, the >> redirection operator can be used to append to existing files. For example, you could use the following command to append a one-line footnote to in.txt:

$ echo "end of list" >> in.txt

Since in.txt already exists, the quote will be appended to the bottom of the existing file. If the file didn’t exist, the >> operator would create ...

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