Linux File Descriptors
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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 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