Difference between revisions of "Linux File Descriptors"
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(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 -...") |
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# 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 | ||
# <> 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 '<' | ||
Line 28: | Line 28: | ||
echo "File was read on: $(date)" >&5 | echo "File was read on: $(date)" >&5 | ||
# Close FD, otherwise will stay open forever | # Close FD, otherwise will stay open forever (aka close file handle) | ||
exec 5>&- | exec 5>&- | ||
</source> | </source> |
Latest revision as of 18:00, 6 November 2019
It's an index table from 0..n per process that indicates what files,pipes,sockets the process has open.
File Descriptors 0,1,2 are reserved for OS, for:
- 0 - STDOUT
- 1 - STDIN
- 2 - STDERR
Following script demonstrates FD usage:
#!/bin/bash echo "File name to read: " read FILE # type 'cities.txt', this file must exist # assign/create FD '5' to a file, what we can refere as a strem to read or write in the future exec 5<>$FILE # open a file for read and write (aka open a file handle) # > 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 (aka close file handle) exec 5>&-
File to read
cat > cities.txt << EOF Osaka Warsaw London EOF