This chapter is from the book
This chapter is from the book
This chapter is from the book
sed
Use the sed utility to make automated modifications to files. The basic format for the sed command is sed 's/RE/string/' file.
The “RE” refers to the term regular expression, a feature that uses special characters to match patterns. See Chapter 15, “Search Text Files Using Regular Expressions,” for more details about regular expressions.
Example of the sed command:
[student@localhost ~]$ head -n 5 /etc/passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin [student@localhost ~]$ head -n 5 /etc/passwd | sed 's/bin/----/' root:x:0:0:root:/root:/----/bash ----:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/s----:/sbin/nologin adm:x:3:4:adm:/var/adm:/s----/nologin lp:x:4:7:lp:/var/spool/lpd:/s----/nologin
sed is a very powerful utility with a large number of features. The following table describes some of the more useful sed utilities:
| Feature | Description |
|---|---|
| '/RE/d' | Deletes lines that match the RE from the output of the sed command. |
| '/RE/c\string' | Changes lines that match the RE to the value of string. |
| '/RE/a\string' | Add string on a line after all lines that match the RE. |
| '/RE/i\string' | Add string on a line before all lines that match the RE. |
The sed command has two important modifiers (characters added to the end of the sed operation):
g—Means “global.” By default only the first RE pattern match is replaced. When the g modifier is used, all replacements are made. See Figure 10.3 for an example.
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Figure 10.3 The g Modifier
i—Means “case-insensitive.” This modifier matches an alpha character regardless of its case. So, the command sed 's/a/-/i' would match either “a” or “A” and replace it with the “-” character.
The sed command can also change the original file (instead of displaying the modified data to the screen). To change the original file, use the -i option.