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Double forloop?

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Top Forums Shell Programming and Scripting Double forloop?
# 8  
Old 01-20-2015
FD is the abbreviation for file descriptor. fd 0 is stdin, fd 1 is stdout, fd2 is stderr. Others are unused by default. So Corona688 uses fd 5 and 6 to open the files, and then redirects the tworeads' stdin to there.
Read e.g. man bash: redirection.
Try - if available - lsof -p$$ to see the open files and fds

Last edited by RudiC; 01-20-2015 at 12:47 PM..
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# 9  
Old 01-20-2015
lol.. and thank you.
For fun i ran lsof on the pid of xorg - because i though bash would be too boring, but now figured i used a /usr/lib64/libdrm_radeon.so.1.0.1 while having a nvidia gpu.

Anyway, it sounds like the numbers of FD could be 'infite' (as in: integer max), but on the other hand, in the use(d range) example here, its always FD numbers less or equal than the available VT's.

So my question is, what is the max number of adress-able file descriptors?
6/7 of which the first two or three are reserverd, or more like 32K'ish?
# 10  
Old 01-20-2015
Without having analyzed the sources: I've seen them up to 255, and in real use up to 74.
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# 11  
Old 01-20-2015
Originally Posted by Corona688
# Open file into FD 5
exec 5<file1
# Open file into FD 6
exec 6<file2

# Read from both files
while read -r LINE <&5 && read -r LINE2 <&6
        echo "Line1 is $LINE1"
        echo "Line2 is $LINE2"

# Close FD 5
exec 5<&-
# Close FD 6
exec 6<&-

This can be further improved if we knew what you were reading -- if you had tokens you needed split etc.
$LINE1 might always be NULL, but $LINE might be valid... <wink>

And thanks, you have shown me how to close a file descriptor...
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# 12  
Old 01-20-2015
Originally Posted by RudiC
Without having analyzed the sources: I've seen them up to 255, and in real use up to 74.

man bash:
"Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally."
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# 13  
Old 01-20-2015
To avoid those conficts, you could use a variable name whose value would be assigned by the shell (man bash):
Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this
case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than or equal to 10 and
assign it to varname. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close.
# 14  
Old 01-20-2015
Furthermore, additional restrictions may apply when using the KornShell: the coprocess facility uses FD 4 and 5 per default to handle the inter-process communication.

I hope this helps.

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