1. Open file descriptors table maps number (file descriptor - FD) into actual
file to which output will be written.
+----+------------+
| FD | File |
+----+------------+
| 0 | /dev/vc/1 |
+----+------------+
| 1 | /dev/vc/1 |
+----+------------+
| 2 | /dev/vc/1 |
+----+------------+
| ... |
2. Redirection operators (shell) affect (change) only 'File' column of the
table. So, if program writes to FD=5 (write(5,..); call) you can _not_
force it to write to another FD with shell redirection - you can only
change the content of 'File' column in the FD=5 row (i.e to what file FD=5
is mapped) and through that change to where output arrives at the end (to
which file).
3. '/dev/stdout' and '/dev/stderr' are _not_ an actual files, they're _links_
to what is _now_ opened at FD=1 and FD=2 correspondingly. I.e links to
file specified in 'File' column of FD=1 and FD=2 rows.
# ls -l /dev/stdout /dev/stderr
lrwxrwxrwx 1 root root 15 Sep 30 07:56 /dev/stderr -> /proc/self/fd/2
lrwxrwxrwx 1 root root 15 Sep 30 07:56 /dev/stdout -> /proc/self/fd/1
4. In construction like
f() {
echo "abc"
return 0
}
res=$(f)
function f() result are sent through pipe to the caller. I think, this is
not clear to say, that result are sent through "stdout", though it's
correct, since "stdout" is _not_ an actual file - it's a link to what is
now opened at file descriptor 1, and now there will be pipe. By default,
when function invoked through command substitution (function will be
executed in a subshell), pipe is opened for writing at FD=1 for child
process (function) and for reading at FD=3 for caller process. So, if you
use something like
echo "abc"
"abc" will be written into pipe (because `echo` always writes to FD=1). But
if you want, that the way how we send result does not affect function's
code, we should move 'pipe' from FD=1 into some unused FD and restore
original FD=1 content.
exec 7>&1 1>&2
When result will be ready, we move 'pipe' back to FD=1 and `echo` result
into it.
exec 1>&7-
echo "result"
Note, that for all child subshells pipe will be opened as well.
Example. Illustrates complete implementation, with several stacked functions
returning result through pipe.
#!/bin/bash
log='./t.log'
read_f='/dev/null'
rm -f $log
f2() {
echo "f2(): SUBSH=$BASH_SUBSHELL" >>$log
echo "f2(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo "f2(): stdout-1: ghi"
echo "f2(): stderr-1: klm" >/dev/stderr
echo "f2(): Before moving pipe somewhere" >>$log && read -n1 < $read_f
eval "exec $save_pipe>&1 1>&$save_stdout-"
echo "f2(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo "f2(): stdout-2: ghi"
echo "f2(): stderr-2: klm" >/dev/stderr
echo "f2(): Before exit f2()" >>$log && read -n1 < $read_f
return 0
}
f() {
echo "f(): SUBSH=$BASH_SUBSHELL" >>$log
echo "f(): \$\$[$$]:" >>$log
lsof -a -p $$ -d '^mem,^cwd,^rtd,^txt' >>$log
echo "f(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo 'f(): stdout-1: abc'
echo 'f(): stderr-1: def' >/dev/stderr
echo "f(): Before moving pipe somewhere" >>$log && read -n1 < $read_f
eval "exec $save_pipe>&1 1>&$save_stdout-"
echo "f(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo 'f(): stdout-2: abc'
echo 'f(): stderr-2: def' >/dev/stderr
echo "f(): Before calling f2()" >>$log && read -n1 < $read_f
eval "exec $save_stdout>&1"
v=$(f2)
echo "-$v-"
eval "exec $save_stdout>&-"
echo "f(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo "f(): Before restoring pipe" >>$log && read -n1 < $read_f
exec >&$save_pipe-
echo 'f(): stdout-3: abc'
echo 'f(): stderr-3: def' >/dev/stderr
echo "f(): BASHPID[$BASHPID]:" >>$log
lsof -a -p $BASHPID -d '^mem,^cwd,^rtd,^txt' >>$log
echo "f(): Before exit f()" >>$log && read -n1 < $read_f
return 0
}
declare -r -i save_stdout=9
declare -r -i save_pipe=7
eval "exec $save_stdout>&1"
v=$(f)
eval "exec $save_stdout>&-"
echo "in main()"
echo "\$\$[$$]:" >>$log
lsof -a -p $$ -d '^mem,^cwd,^rtd,^txt' >>$log
echo "-$v-"
Here is illustration:
(subshell)
main() ....................
+--------------+ . f() .
| 3r pipe | call f() . +--------------+ .
| 9u "stdout" |----------->| 1w pipe(f) | .
+--------------+ . | 9u "stdout" | .
. +--------------+ .
. | .
. | move pipe(f)
. | .
. v . (subshell)
. +--------------+ . ....................
. | 1w "stdout" | . . f2() .
. | 7w pipe(f) | call f2() . +--------------+ .
. | 9- (closed) |------------>| 1w pipe(f2) | .
. +--------------+ . . | 7w pipe(f) | .
. . . | 9u "stdout" | .
. . . +--------------+ .
. . . | .
. . . | move pipe(f2)
. . . | over pipe(f)
. . . | .
. . . v .
. . . +--------------+ .
. +--------------+ . return . | 1w "stdout" | .
. | 1w "stdout" |<------------| 7w pipe(f2) | .
. | 7w pipe(f) | . . | 9- (closed) | .
. +--------------+ . . +--------------+ .
. | . ....................
. | restore pipe(f)
. | .
. v .
. +--------------+ .
+--------------+ return . | 1w pipe(f) | .
| 1u "stdout" |------------| 7- (closed) | .
+--------------+ . +--------------+ .
....................
And here is log (slightly edited)
# ./t.sh >|./1.tmp
f(): SUBSH=1
f(): $$[4794]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4794 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4794 root 1w REG 8,7 0 1121662 .../1.tmp
t.sh 4794 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4794 root 3r FIFO 0,6 14132 pipe
t.sh 4794 root 9w REG 8,7 0 1121662 .../1.tmp
t.sh 4794 root 255r REG 8,7 2075 1121653 .../t.sh
f(): BASHPID[4796]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4796 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 1w FIFO 0,6 14132 pipe
t.sh 4796 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 9w REG 8,7 0 1121662 .../1.tmp
f(): Before moving pipe somewhere
f(): BASHPID[4796]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4796 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 1w REG 8,7 0 1121662 .../1.tmp
t.sh 4796 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 7w FIFO 0,6 14132 pipe
f(): Before calling f2()
f2(): SUBSH=2
f2(): BASHPID[4803]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4803 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4803 root 1w FIFO 0,6 14194 pipe
t.sh 4803 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4803 root 7w FIFO 0,6 14132 pipe
t.sh 4803 root 9w REG 8,7 19 1121662 .../1.tmp
f2(): Before moving pipe somewhere
f2(): BASHPID[4803]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4803 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4803 root 1w REG 8,7 19 1121662 .../1.tmp
t.sh 4803 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4803 root 7w FIFO 0,6 14194 pipe
f2(): Before exit f2()
f(): BASHPID[4796]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4796 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 1w REG 8,7 61 1121662 .../1.tmp
t.sh 4796 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 7w FIFO 0,6 14132 pipe
f(): Before restoring pipe
f(): BASHPID[4796]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4796 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4796 root 1w FIFO 0,6 14132 pipe
t.sh 4796 root 2u CHR 4,1 5466 /dev/vc/1
f(): Before exit f()
$$[4794]:
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
t.sh 4794 root 0u CHR 4,1 5466 /dev/vc/1
t.sh 4794 root 1w REG 8,7 71 1121662 .../1.tmp
t.sh 4794 root 2u CHR 4,1 5466 /dev/vc/1
t.sh 4794 root 255r REG 8,7 2075 1121653 .../t.sh
четверг, 30 сентября 2010 г.
[draft][part] Shell I/O redirection
DISCLAIMER. English language used here only for compatibility (ASCII only), so any suggestions about my bad grammar (and not only it) will be greatly appreciated.
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