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Any combination of the following exec commands can be used in your code to open a file and assign it a single-digit file descriptor: exec 3> file_1: Opens file_1 for output in overwrite mode, which would then be accessed with file descriptor 3. exec 4< file_2: Opens file_2 for input in read mode, which would then be accessed with file descriptor 4. exec 5>> file_3: Opens file_3 for output in append mode, which would then be accessed with file descriptor 5. exec 6<> file_4: Opens file_4 for both reading and writing, which would then be accessed with file descriptor 6. Unlike the other examples, where you must access the files in the same way the descriptors were opened, for either reading or writing, this type of descriptor can be accessed in both ways. Once the file descriptors are open, you can access them with various input or output statements. To access a specific open file descriptor, complete an expression with the syntax >&fd, where fd denotes the single-digit descriptor. Here are some examples of reading and writing to open files:
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echo "The quick brown fox jumped over the lazy dog" >&3 grep 172.16 /etc/hosts >&5 read line <&4
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I have matched the file-descriptor numerals (relating to input and output) with the exec commands that opened them originally. Using the exec syntax you can also specify your output at runtime instead of hardcoding it. The following example demonstrates the technique:
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If [ -n "$DEBUG" ] then exec 5>&1 else exec 5> $LOGFILE fi echo "Some Text" >&5
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Based on the value of the DEBUG variable, the code sends output from file descriptor 5 either to stdout or to a log file.
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When file access is complete, you should close the open file with the exec command, as shown here:
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exec 3>&-
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This keeps your code clean. In both bash and ksh you will receive a Bad file descriptor error if you attempt to access a file descriptor that hasn t been opened. There is no trouble with closing a file descriptor that is not already open.
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Descriptor Access from the Shell
One other method of reading from and writing to an open file descriptor is to use the shell s built-in print and read commands. Only ksh has the print command, but both ksh and bash have read. Both of these built-in commands take the -u switch, which gives you the ability to specify the file descriptor you want to access. Once the file is open, you can output directly to or read directly from that specific descriptor using these built-in commands. Two methods can be used with the print command in ksh. The first of the two examples here is the redirection to a specific file descriptor, as already discussed. The second uses the -u switch with a single-digit file descriptor. These are equivalent commands and are available only in ksh.
print "all your base are belong to us" >&3 print -u3 "Now is the time"
For bash, the available command is very similar. You have access to only the redirection syntax, however, as the -u switch isn t supported. You also need to replace the print command with echo.
echo "all your base are belong to us" >&3
Like the built-in print command, the read command can be used in two ways. The first of the following two examples specifies redirection to a file descriptor. The second uses the -u switch, like the print command in ksh. The main difference here is that both bash and ksh have this capability.
read line <&3 read -u3 line
All of these examples assume the file has been opened for reading or writing as appropriate. When you try to access a file descriptor that has not been opened, you will receive an invalid file descriptor: Bad file descriptor error in bash and a bad file unit number [Bad file descriptor]: error in ksh.
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