Operators

Order of redirection matters

Note that stdout (fd 1) is used by default if none is specified

Eg. > is equivalent to 1>

• The following directs both stdout (fd 1) and stderr (fd 2) to the file dirlist

ls > dirlist 2>&1 

• While the following directs only the stdout (fd 1) to the file dirlist

  • The stderr (fd 2) still points to terminal (orginal value of stdout)

ls 2>&1 > dirlist

The reason being that the shell interprets the values from left to right.

Steps in the first example

1. > dirlist: Redirect stdout to dirlist file

2. 2>&1: Redirect stderr (fd 2) to stdout (&1, fd 1)

• Since stdout is pointing to the dirlist file, this points stderr to that file too

Steps in the second example

1. 2>&1: Redirect stderr (fd 2) to stdout (fd 1)

• However, since stdout is currently pointing to the terminal, the stderr will point to it too

1. > dirlist: Redirect stdout to the dirlist file

• Overall, stderr points to the terminal, while stdout points to the dirlist file

Operators

1. >: overwrite data in file

$ cat list.txt
bye

$ echo "hello" > list.txt

$ cat list.txt
hello

• Notice that the text bye is replaced by hello

1. >> : append data to file

$ cat list.txt
bye

$ echo "hello" >> list.txt

$ cat list.txt
bye 
hello

• Notice that the text hello is appended to the file without overwriting the text bye

1. <: read from file

$ cat list.txt
bye
hello

$ wc -w < list.txt
2

1. Pipe (|): redirect stdout from one process to the stdin of another

$ cat list.txt | wc -w
2