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The following are some of the most commonly used redirection arguments and their meanings. Additional ones and more detailed explanations are specified in the bash and ksh man pages. expression < file: Redirect the contents of file into the expression. An example of this usage is the command mail -s "/etc/hosts" you@domain.com < /etc/hosts, in which the /etc/hosts file will be used as the body of the mail message being sent. expression > file: Redirect the output of expression to the specified file. If the file does not exist, it is created. If the file already exists, it is overwritten unless the noclobber option to set is applied. An example is grep 192.168 /etc/hosts > /tmp/outfile, where /tmp/outfile will contain the results of the grep command after the line finishes executing.
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CHAPTER 9 DATA REDIRECTION
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expression >| file: Redirect the output of expression to the specified file. This is similar to the previous example, except that it overrides the noclobber option of the shell. When the noclobber option is set, the file won t be overwritten when the > redirector is used. This syntax forces that overwrite. expression >> file: Redirect the output of expression to the specified file in append mode. If the command in the earlier example (grep 192.168 /etc/hosts > /tmp/ outfile) is followed by the command grep 172.16 /etc/hosts >> /tmp/outfile, then the output of this grep command will be appended to the end of the /tmp/outfile file. expression 2> /dev/null > file: Redirect stderr output from the expression to /dev/null while redirecting stdout output to the specified file. expression > /dev/null 2> file: Redirect stdout from the expression to /dev/null while redirecting stderr to the file. This is the converse of the preceding example. expression > file 2>&1: Redirect both stdout and stderr output from expression to the specified file. This syntax specifies that stderr (2) output is assigned to stdout (1) and both are redirected to the specified file. expression &> file: Redirect both stdout and stderr output from the expression to the file. This is a shorthand version of the previous syntax (expression > file 2>&1). This is a feature of the bash shell. expression 2> file: Redirect stderr output from the expression to the file. An example of this is find /home -type f -exec grep -l "some_string" {} \; 2> /dev/null. This command will output all the filenames returned by the find command containing the string some_string, but redirect any permission denied errors to /dev/null. Redirection can be used not only for simple commands and script invocations, but also with if, for, while, until, and case structures, as well as pipes and batched commands. The following code segments give a few samples of how you might use redirection of complex statements in your scripts:
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{ cat file1 cat file2 cat file3 } > file.output cat nodelist | while read line ; do echo $node rsh $node uptime done >> uptime.report
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while read line do count=$((count+1)) echo -n Field 2 of line $count is: echo $line | awk '{print $2}' done < /input/file count=0 while [ $count -lt 10 ] do echo $count count=$((count+1)) done > count.output
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Multiple redirections can also be used. In the following short example the code takes the contents of the /etc/hosts file as input to the while loop:
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count=0 while read line do count=$((count+1)) echo $count: $line done < /etc/hosts > lined.hosts
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The loop adds line numbers to the file and the final output is then redirected to the lined.hosts file.
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Access to User-Specified File Handles
In most programming languages, you can open a file descriptor for reading, writing, or both, then read or make modifications to the data and close the file. Shell scripting languages are no exception. Up to now, I have discussed redirection only of output to a given file or to one of the standard input or output file streams. Now I ll discuss the method of opening a file handle for a specific type of operation (read/write/both), but instead of opening the file each time you want to access it, you create a file handle with a userdefined file descriptor that can then be accessed in various modes, using the same syntax as the standard input/output descriptors. You create a handle for a specified file via the exec command. When opening the file, you give it a single-digit file descriptor in the range of 3 9 to avoid conflict with the standard stream descriptors of 0 2.
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