Redirection Operators for Testers

Redirection operators (|, >, >>, <) let you chain Unix commandline commands together, and move information in and out of the chain.

• | chains i.e. sends the output of one to the input of the next.
• < sends input into commands – so you can (generally) doathing < file.txt (which is similar to the common chain cat file.txt | doathing )
• > and >> redirect the output to a file (> overwrites while >> appends)
• 2> redirects the standard error (output) to a file

roughs below

more

> == 1> and directs stdout to overwrite something
2> directs stderr
>> and appends rather than overwrites. But 2>>&1 is bad syntax
The pattern 2>&1 sends errors to stdout – order is not guaranteed: streams may be interleaved, but one does not overwrite the other. 2>>&1 appends to what is there – so what if ls > dirlist 2>>&1? While >and>>are related,|(pipe) and||(OR) are not.&(file descriptor) and&&` (AND) are not, either.

file descriptors

The & symbol (e.g., 2>&1) refers to numbered file descriptors for input/output streams – or for files if opened explicitly. By default:

FD 0 represents standard input (stdin),FD 1 represents standard output (stdout),FD 2 represents standard error (stderr) 
Higher numbers only make sense if they’ve been opened explicitly