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中国DOS联盟论坛 » DOS批处理 & 脚本技术（批处理室） » [Original] Simulating command-line handle operations (to help understand handle copying and backup)
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Posted 2009-06-12 11:55 · 中国 山西 运城 电信
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Floor 2
Posted 2009-06-13 12:36 · 中国 北京 联通
So how do you tell what a handle points to?
echo 1&handle number to test is con
If con is displayed, then the handle value is con.
Otherwise it is nul or a file.
I didn't understand this part.
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Posted 2009-06-13 21:11 · 中国 山西 运城 电信
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A typo correction:
echo 1&handle number to test is con
should be
echo 1<&handle number to test is con
It is equivalent to
echo is con 1<&handle number
Example analysis:
1<&7
means copying what handle 7 points to into handle 1.
And the output of echo is sent through handle 1 by default.
So if handle 7 points to con,
after copying it to 1, then 1 also points to con.
Then the output of echo will appear on the screen.
Otherwise it will not appear on the screen.
Of course, this is only a simple way to tell con from non-con.
For more specific methods, you'll still need to do more testing yourself.
[ Last edited by qzwqzw on 2009-6-13 at 21:14 ]
echo 1&handle number to test is con
should be
echo 1<&handle number to test is con
It is equivalent to
echo is con 1<&handle number
Example analysis:
1<&7
means copying what handle 7 points to into handle 1.
And the output of echo is sent through handle 1 by default.
So if handle 7 points to con,
after copying it to 1, then 1 also points to con.
Then the output of echo will appear on the screen.
Otherwise it will not appear on the screen.
Of course, this is only a simple way to tell con from non-con.
For more specific methods, you'll still need to do more testing yourself.
[ Last edited by qzwqzw on 2009-6-13 at 21:14 ]
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| s11ss | +3 | 2009-06-14 04:30 |
Floor 4
Posted 2009-06-14 04:32 · 中国 北京 联通
Originally posted by qzwqzw at 2009-6-13 09:11 PM:
A typo correction:
echo 1&handle number to test is con
should be
echo 1<&handle number to test is con
It is equivalent to
echo is con 1<&handle number
Example analysis:
1<&7
means ...
<& seems to be the same as >&, both copy the latter handle to the former handle.
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Posted 2009-06-14 21:18 · 中国 山西 电信
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Now I found that the only difference between >& and <&
is the default value on the left side.
The default left value of >& is 1.
The default left value of <& is 0.
The resulting effect is that after
echo. >nul 3>nul makes handle 1 become nul,
echo Hello Handle <&2 has no output
echo Hello Handle >&2 has output
is the default value on the left side.
The default left value of >& is 1.
The default left value of <& is 0.
The resulting effect is that after
echo. >nul 3>nul makes handle 1 become nul,
echo Hello Handle <&2 has no output
echo Hello Handle >&2 has output
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| s11ss | +3 | 2009-06-14 23:19 |
Floor 6
Posted 2009-06-14 23:11 · 中国 北京 联通
Originally posted by qzwqzw at 2009-6-14 09:18 PM:
Now I found that the only difference between >& and <&
is the default value on the left side.
The default left value of >& is 1.
The default left value of <& is 0.
The resulting effect is that after
echo. >nul 3>nul ...
hh ntcmds.chm::/redirection.htm
If no handle is defined, the default handle for the < redirection input operator is 0, and the default handle for the > redirection output operator is 1.
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