[SOLVED] Bash read/write file descriptors -- seek to start of file

Issue

I tried to use the read/write file descriptor in bash so that I could delete the file that the file descriptor referred to afterward, as such:

F=$(mktemp)
exec 3<> "$F"
rm -f "$F"

echo "Hello world" >&3
cat <&3

but the cat command gives no output. I can achieve what I want if I use separate file descriptors for reading and writing:

F=$(mktemp)
exec 3> "$F"
exec 4< "$F"
rm -f "$F"

echo "Hello world" >&3
cat <&4

which prints Hello world.

I suspected that bash doesn't automatically seek to the start of the file descriptor when you switch from writing to reading it, and the following combination of bash and python code confirms this:

fdrw.sh

exec 3<> tmp
rm tmp

echo "Hello world" >&3
exec python fdrw.py

fdrw.py

import os  

f = os.fdopen(3)
print f.tell()
print f.read()

which gives:

$ bash fdrw.sh
12

$ # This is the prompt reappearing

Is there a way to achieve what I want just using bash?

Solution

No. bash does not have any concept of "seeking" with its redirection. It reads/writes (mostly) from beginning to end in one long stream.

Answered By - Ignacio Vazquez-Abrams

Answer Checked By - Cary Denson (WPSolving Admin)