Shell practice: Introduction to the sed stream editor

Quick Edit

Adding and Inserting

You add lines in a file by stating a line number or after (a) and before (i) text found with a search pattern (Figure 21). If you enter multiple line numbers or the pattern matches multiple times, the insertion occurs for each instance.

Figure 2: Searching for uppercase (Man) or lowercase (man).

Figure 21: Adding lines.

In the first sed command, a new line is added above the first line in the file; in the next command, it's added at the end ($). The command at the next prompt adds a new line above the matched search pattern, and the next line adds it below.

Shell Variables

If a shell variable needs to be resolved, you need to enclose the statements in double quotes (") instead of single quotes ('). The short shell script in Listing 3 shows how to handle variables by searching through the sample file and outputting the matching lines. Figure 22 shows the result.

Listing 3

searchString.sh

01 #! /bin/sh
02 echo -n "Enter search string: ";read sstring
03 cat textdata.txt | sed -n "/$sstring/"p

Figure 22: Handling variables in Bash scripts.

Conclusion

With sed, you can execute complex text manipulation commands without intervention. Its cryptic syntax encourages building scripts one bit at a time.

Infos

Project page for sed: http://sed.sourceforge.net/
Text files: ftp://ftp.linux-magazine.com/pub/listings/admin-magazine.com/28

The Author

Harald Zisler has focused on FreeBSD and Linux since the early 1990s. He is the author of various articles and books on engineering and IT topics.

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