[Happy BASH] Assigns the file descriptor (fd) to file for output

File descriptors 0, 1 and 2 are reserved for stdin, stdout and stderr respectively. However, bash shell allows you to assign a file descriptor to an input file or output file. This is done to improve file reading and writing performance. This is known as user defined file descriptors.

Syntax

You can assign a file descriptor to an output file with the following syntax:

exec fd> output.txt

• where, fd >= 3

Example

Create a shell script called fdwrite.sh:

#!/bin/bash
# Let us assign the file descriptor to file for output
# fd # 3 is output file 
exec 3> /tmp/output.txt
 
# Executes echo commands and  # Send output to 
# the file descriptor (fd) # 3 i.e. write output to /tmp/output.txt
echo "This is a test" >&3
 
# Write date command output to fd # 3
date >&3
 
# Close fd # 3
exec 3<&-

Save and close the file. Run it as follows:

chmod +x fdwrite.sh
./fdwrite.sh
cat /tmp/output.txt

Note:

1. "3>" 必須連在一起

2. ">&" 必須連在一起

3. "3<&" 必須連在一起