`

The awk command is a super powerful tool, and we encourage you to dig

deeper into its capabilities by running man awk for more information.

Editing Streams with sed

The sed (stream editor) command takes actions on text. For example, it can

replace the text in a file, modify the text in some command’s output, and even

delete selected lines from files.

Let’s use sed to replace any mentions of the word Mozilla with the word

Godzilla in the log.txt file. We use its s (substitution) command and g (global)

command to make the substitution across the whole file, rather than to just the first

occurrence (Listing 2-31).

$ sed 's/Mozilla/Godzilla/g' log.txt

Listing 2-31

Replacing a string with another string

This will output the modified version of the file but won’t change the original

version. You can redirect the output to a new file to save your changes:

$ sed 's/Mozilla/Godzilla/g' log.txt > newlog.txt

We could also use sed to remove any whitespace from the file with the /

// syntax, which will replace whitespace with nothing, removing it from the

output altogether (Listing 2-32).

$ sed 's/ //g' log.txt

Listing 2-32

Removing whitespace with sed

If you need to delete lines of a file, use the d command. In Listing 2-33, 1d

deletes (d) the first line (1).

$ sed '1d' log.txt

Listing 2-33

Deleting the first line with sed

To delete the last line of a file, use the dollar sign ($), which represents the

last line, along with d (Listing 2-34).

$ sed '$d' log.txt

Listing 2-34

Deleting the last line with sed

You can also delete multiple lines, such as line 5 and 7 (Listing 2-35).

$ sed '5,7d' log.txt

Listing 2-35

Deleting lines 5 and 7

Lastly, you can print specific line ranges, such as lines 2 through 15 (Listing

2-36).

$ sed -n '2,15 p' log.txt

Black Hat Bash (Early Access) © 2023 by Dolev Farhi and Nick Aleks