Linux File Descriptors - Revision history

Pio2pio at 16:00, 6 November 2019

2019-11-06T16:00:35Z

← Older revision		Revision as of 18:00, 6 November 2019
Line 15:		Line 15:

	# assign/create FD '5' to a file, what we can refere as a strem to read or write in the future		# assign/create FD '5' to a file, what we can refere as a strem to read or write in the future
	exec 5<>$FILE # open a file for read and write		exec 5<>$FILE # open a file for read and write (aka open a file handle)
	# > open for write		# > open for write
	# < open for read		# < open for read
Line 28:		Line 28:
	echo "File was read on: $(date)" >&5		echo "File was read on: $(date)" >&5

	# Close FD, otherwise will stay open forever		# Close FD, otherwise will stay open forever (aka close file handle)
	exec 5>&-		exec 5>&-
	</source>		</source>

Pio2pio at 15:59, 6 November 2019

2019-11-06T15:59:41Z

← Older revision		Revision as of 17:59, 6 November 2019
Line 20:		Line 20:
	# <> open for rw		# <> open for rw

	while read -r CITY; do		while read -r CITY; do # read a file line-by-line
	echo "City name: $CITY"		echo "City name: $CITY"
	done <&5 # instead redirecting a file value, we redirect a FD for reading '<'		done <&5 # instead redirecting a file value, we redirect a FD for reading '<'

Pio2pio: Created page with "It's an index table from 0..n per process that indicates what files,pipes,sockets the process has open. File Descriptors 0,1,2 are reserved for OS, for: * 0 - STDOUT * 1 -..."

2019-11-06T15:58:47Z

Created page with "It's an index table from 0..n per process that indicates what files,pipes,sockets the process has open. File Descriptors 0,1,2 are reserved for OS, for: * 0 - STDOUT * 1 -..."

New page

It's an index table from 0..n per process that indicates what files,pipes,sockets the process has open.

File Descriptors 0,1,2 are reserved for OS, for:
* 0 - STDOUT
* 1 - STDIN
* 2 - STDERR

Following script demonstrates FD usage:
<source lang=bash>
#!/bin/bash
echo "File name to read: "
read FILE # type 'cities.txt', this file must exist

# assign/create FD '5' to a file, what we can refere as a strem to read or write in the future
exec 5<>$FILE # open a file for read and write
# > open for write
# < open for read
# <> open for rw

while read -r CITY; do
echo "City name: $CITY"
done <&5 # instead redirecting a file value, we redirect a FD for reading '<'
# &<number> - ampersand indicates that it's FD

# Write to a file but instead using a file we use FD for writting '>'
echo "File was read on: $(date)" >&5

# Close FD, otherwise will stay open forever
exec 5>&-
</source>

File to read
<source lang=bash>
cat > cities.txt << EOF
Osaka
Warsaw
London
EOF
</source>