Linux File Descriptors

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:

#!/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 # read a file line-by-line
  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>&-

File to read

cat > cities.txt << EOF
Osaka
Warsaw
London
EOF