# Short answer:

command > filename    # Redirect stdout to file "filename", overwrite if present

command 1> filename   # Redirect stdout to file "filename" (same as >)

command >> filename   # Redirect and append stdout to file "filename"

command 2> filename   # Redirect stderr to file "filename"

command 2>> filename  # Redirect and append stderr to file "filename"

command &> filename   # Redirect both stdout and stderr to file "filename"

command &>> filename  # Redirect and append both stdout and stderr to file "filename"

command < filename    # Send contents of file "filename" to command (redirect stdin)

command | command_2   # Redirect (pipe) stdout of command to command_2

​

# Long answer:

# First, it's helpful to know about file descriptors (fds). A fd is a number 

#   that the operating system assigns to an open file to keep track of it.

#   Consider it a simplified type of file pointer. It is analogous to a file

#   handle in C.

# In bash, the three default file descriptors are:

#   0, which refers to the standard input (stdin),

#   1, which refers to the standard output (stdout), and

#   2, which refers to the standard error (stderr)

# Redirection allows commands’ fds to be duplicated, opened, closed, made to

#   refer to different files, and can change the files the command reads from

#   and writes to. Redirection may also be used to modify file handles in the

#   current shell execution environment

# Note, redirections are processed in the order they appear, from left to right

# Note, if the fd isn't specified during a redirection, one is usually assumed

#   e.g. if you use >, it assumes an fd of 1, identical to: 1>

​

# Given this background, there are two main redirection formats to be aware of:

i>j

#   Here, "i" is a file descriptor and "j" is a filename

#   File descriptor "i" is redirected to file "j"

i>&j

#   Here, "i" is still a file descriptor but the & makes "j" another file

#       descriptor. This syntax causes fd i to be redirected to fd j. In other

#       words, all output of the file or command pointed to by i gets sent to

#       the file pointed to by j

​

# Example usage:

ls -ld /tmp /tnt

-> ls: cannot access /tnt: No such file or directory # stderr

-> drwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp # stdout

​

ls -ld /tmp /tnt > /dev/null # redirect stdout to trash

-> ls: cannot access /tnt: No such file or directory # stderr only

​

ls -ld /tmp /tnt 2> /dev/null # redirect stderr to trash

-> drwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp # stdout

​

# The following differences in behavior is slightly confusing to me. The only

#   way I can make sense of it is if in the top example, stdout is first

#   redirected to trash and then stderr is redirected to stdout which is still

#   pointing to the trash, causing it to be discarded as well. In the bottom

#   example, it would seem like stderr is first sent to stdout (which gets

#   printed to the console) and then the stdout gets redirected to the trash,

#   and therefore isn't printed

ls -ld /tmp /tnt > /dev/null 2>&1

​

ls -ld /tmp /tnt 2>&1 > /dev/null

-> ls: cannot access /tnt: No such file or directory # stderr

​

# See these links for more on redirection:

https://www.gnu.org/software/bash/manual/html_node/Redirections.html

https://stackoverflow.com/questions/818255/in-the-shell-what-does-21-mean