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Using sed Using awk
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Let s begin by looking at sed
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Using sed
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The sed command is a stream text editor Unlike the interactive text editors that you ve already learned how to use in this book, such as vi, a stream editor takes a stream of text as its stdin and then performs operations on it that you specify Then, sed sends the results to stdout, which is to the screen by default You can use the following commands with sed:
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Replaces instances of a specified text string with another text string The syntax for using the s command is sed s/term1/term2/ For example, in Figure 9-36, I ve used the cat command to display a file in the tux user s home directory named test2txt I then use cat to read test2txt and then
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Manage Shell Command Inputs and Outputs
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FIGURE 9-36
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pipe the stdout to the stdin of the sed command and specify that the term reque be replaced with REQUE
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Deletes the specified text For example, to delete every line of text from the stdin that contains the term eos, you would enter sed /eos/d This is shown in Figure 9-37
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FIGURE 9-37
Using sed to delete lines of text
9: Using the Linux Shell
Remember, sed doesn t actually modify the source of the information in this case, the test2txt file It takes its stdin and makes the changes and sends it to the stdout If you want to save the changes made by sed, then you need to redirect its stdout to a file using > The sed and awk commands discussed in this chapter are quite powerful We only have space here to cover the basics you need for your Linux+ exam You should review the man page for both of these commands and use this information to explore the possibilities available In addition to sed, you can also use awk to manipulate output Let s discuss how this is done next
Using awk
Like grep and sed, awk can be used to receive output from another command as its stdin and manipulate it in a manner you specify However, the way awk does this is a little bit different The awk command treats each line of text it receives as a record Each word in the line, separated by a space or tab character is treated as a separate field within the record For example, consider the text file displayed by the cat command shown in Figure 9-38 According to awk, this file has three records because it has three separate lines of text Each line of text has a carriage return/linefeed character at the end that creates a new line This is the character awk uses to define the end of a record The first record has 14 fields; the second record also has 14 fields The last record has only 3 fields These are shown in Table 9-3
FIGURE 9-38
Viewing the test2txt file
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TABLE 9-3
Viewing a Text File as a Database
Field10
Field11
Field12 vis ut,
1 2 3
Et At democritum
reque
everti
atomorum partem,
vel, mazim
eos mandamus
sale at
partiendo proUsu
id, agam
quaeque legere
salutandi delicata
at per
vis cibo scriptorem an
Notice that white space, not punctuation, delimits the fields Each field is referenced as $field_number For example, the first field of any record is referenced as $1, the second as $2, and so on Using awk, we can specify a field in a specific record and manipulate it in some manner The syntax for using awk is awk pattern {manipulation} For example, we could enter cat /test2txt |awk {print $1,$2,$3} to print out the first three words ( fields ) of each line ( records ) Because we didn t specify a pattern to match on, awk simply prints out the first three words of every line This is shown in Figure 9-39 You can also include a pattern to specify exactly which records to search on For example, suppose we only wanted to display the first three fields of any record that includes the text em somewhere in the line To do this, you add a pattern of /em/ to the command This is shown in Figure 9-40
FIGURE 9-39
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