Shell Scripting Learning Summary--file descriptors and redirects

The file descriptor is an integer associated with the input and output of the file, which is used to track open files, and the file descriptor 0,1,2 is reserved by the system.

0--stdin (standard input)

1--stdout (standard output)

2--stderr (standard error)

echo a1>CP A1 A2; CP  Cat A *cat: a1:permission denieda1a1

Direct 0 to Std.txt, 2 to Err.txt

$ cat A * >std.txt 2>err.txt

Both 0 and 1 are directed to the OUT.txt

$ cat A * >out.txt 2>&1.txt

Tee command

　　Redirects the stdin to text and prints it on the screen.

Cat a*| Tee OUT.txt Cat  cat  out.txt a1a1

Note: OUT.txt does not contain "cat: A1:permission denied" because it can only be read from stdin.

The default tee command overwrites the file, but provides a-a option to append.

Cat a*| Tee-a out.txt

Text block inside the redirect script

#/bin//nolog <<eof >log.txtconn Scott/Tigerselectcat; Commit;exiteof

The content between <<eof >log.txt and EOF will be redirected to Log.txt as Stdin,stdout.

Custom file descriptors

• Create a file descriptor to write to

4>echo newline >&4cat  output.txt newline

• Creates a file descriptor for reading

echo Hello >
3<input.txtcat <&3Hello

Shell Scripting Learning Summary--file descriptors and redirects