Created by Peteris Krumins ([email protected]) www.catonmat.net -- good coders code, great coders reuse
| Redirection | Description |
|---|---|
cmd > file |
Redirect the standard output (stdout) of cmd to a file. |
cmd 1> file |
Same as above. 1 is the default file descriptor for stdout. |
cmd 2> file |
Redirect the standard error (stderr) of cmd to a file. 2 is the default file descriptor for stderr. |
cmd >> file |
Append stdout of cmd to a file. |
cmd 2>> file |
Append stderr of cmd to a file. |
cmd &> file |
Redirect stdout and stderr to a file. |
cmd > file 2>&1 |
Another way to redirect both stdout and stderr of cmd to a file. This is not the same as cmd 2>&1 > file. Redirection order matters! |
cmd > /dev/null |
Discard stdout of cmd. |
cmd 2> /dev/null |
Discard stderr of cmd. |
cmd &> /dev/null |
Discard stdout and stderr. |
cmd < file |
Redirect the contents of the file to the stdin of cmd. |
cmd << EOL ... EOL |
Here-doc syntax - Redirect the text between the EOL markers to the stdin of cmd |
cmd <<- EOL ... EOL |
Here-doc syntax - ignore leading tabs |
cmd <<< "string" |
Here-string syntax - same as here-doc but without EOL markers |
exec 2> file |
Redirect stderr of all commands to a file |
exec 3< file |
Open a file for reading using a custom file descriptor. |
exec 3> file |
Open a file for writing using a custom file descriptor. |
exec 3<> file |
Open a file for reading and writing using a custom file descriptor. |
exec 3>&- |
Close a file descriptor. |
exec 4>&3 |
Make file descriptor 4 to be a copy of file descriptor 3. (Copy fd 3 to 4.) |
exec 4>&3- |
Copy file descriptor 3 to 4 and close fd 3. |
echo "foo" >&3 |
Write to a custom file descriptor. |
cat <&3 |
Read from a custom file descriptor. |
(cmd1; cmd2) > file |
Redirect stdout from multiple commands to a file (using a sub-shell). |
{ cmd1; cmd2; } > file |
Redirect stdout from multiple commands to a file (faster; not using a sub-shell). |
cmd <(cmd1) |
Redirect output of cmd1 to an anonymous fifo, then pass the fifo to cmd as an argument. Useful when cmd doesn't read from stdin directly and expects a file. |
cmd < <(cmd1) |
Redirect stdout of cmd1 to an anonymous fifo, then redirect the fifo to stdin of cmd. example: `diff <(find /path1 | sort) <(find /path2 | sort) |
cmd <(cmd1) <(cmd2) |
Redirect stdout of cmd1 cmd2 to two anonymous fifos, then pass both fifos as arguments to cmd |
cmd1 >(cmd2) |
Run cmd2 with its stdin connected to an anonymous fifo, and pass the filename of the pipe as an argument to cmd1. |
cmd1 | cmd2 |
Redirect stdout of cmd1 to stdin of cmd2. Pro-tip: This is the same as cmd1 > >(cmd2), same as cmd2 < <(cmd1), |
cmd1 | & cmd2 |
Redirect stdout and stderr of cmd1 to stdin of cmd2 (bash 4.0+ only). Use cmd1 2>&1 | cmd2 for older bashes. |
cmd | tee file |
Redirect stdout of cmd to a file and print it to screen. |
exec {filew}> file |
Open a file for writing using a named file descriptor called {filew} (bash 4.1+) |
cmd 3>&1 1>&2 2>&3 |
Swap stdout and stderr of cmd. |
cmd > >(cmd1) 2> >(cmd2) |
Send stdout of cmd to cmd1 and stderr of cmd to cmd2. |
- A File Descriptor (FD) is a positive integer which refers to an open file.
- Every process inherits three open FDs from its parent: 0 "standard input"; 1 "standard output"; and 2 "standard error".
- A process that is started without one or more of these may behave unpredictably. (So never close stderr. Always redirect to /dev/null instead.)
Redirection involves changing the default File Descriptors to somewhere else:
A common example:
Redirect both stdout and stderr to a file...
some_command 2>&1 > some_file WRONG!
some_command >some_file 2>&1 CORRECT!
Why is the first one wrong? Because order matters. It goes left to right.
The first one reads: redirect stderr to where stdout is pointing right
now (the screen), then redirect stdout to a file.
The second one reads: redirect stdout to a file, then redirect stderr to
where stdout is pointing right now (the file).