Redirect Operator

The redirect operator > connects a program’s output to a file instead of the terminal. By default, a process has three standard streams, each identified by a File Descriptor:

FD	Name	Default
0	stdin	keyboard
1	stdout	terminal
2	stderr	terminal

Operators

Bash	PowerShell	Effect
>	>	Redirect stdout to file (overwrite)
>>	>>	Redirect stdout to file (append)
<	Get-Content |	Redirect file to stdin
2>	2>	Redirect stderr to file
2>>	2>>	Redirect stderr to file (append)
&>	*>	Redirect both stdout and stderr to file
2>&1	2>&1	Redirect stderr to wherever stdout is going
1>&2	1>&2	Redirect stdout to wherever stderr is going

The number before > selects which file descriptor to redirect. & on the right side means “file descriptor”, not “file named 1”. So 2>&1 says “send FD 2 (stderr) to the same place as FD 1 (stdout).”

Without &, 2>1 would redirect stderr to a file literally named 1.

Combining with Pipes

A Pipe | only passes stdout to the next program. To also capture stderr through a pipe, redirect it into stdout first:

program 2>&1 | grep "error"

Order matters. The shell processes redirections left to right:

# stderr goes to stdout, then stdout goes to file
program > output.txt 2>&1    # file gets both stdout and stderr
 
# stdout goes to file, then stderr goes to original stdout (terminal)
program 2>&1 > output.txt    # file gets only stdout, stderr still on terminal

Tee

tee reads from stdin and writes to both a file and stdout simultaneously, letting you save output while still seeing it.

# Capture both stdout and stderr to file, and still see it in the terminal
program 2>&1 | tee output.txt
 
# Append instead of overwrite
program 2>&1 | tee -a output.txt

# Capture both stdout and stderr to file, and still see it in the terminal
program 2>&1 | Tee-Object -FilePath output.txt
 
# Append instead of overwrite
program 2>&1 | Tee-Object -FilePath output.txt -Append

/dev/null

Redirect to /dev/null to silently discard output:

program > /dev/null 2>&1    # discard everything
program 2> /dev/null        # discard only errors