`

You may sometimes find it useful to redirect the standard error

stream to a file, as we’ve done here, so you can log any errors that

occur during runtime. For example, the next example runs a non-

existent command lzl. This should generate bash errors that will be

written into the error.txt file.

$ lzl 2> error.txt

$ cat error.txt

bash: lz: command not found

Listing 1-13

Redirecting the standard error stream using its file descriptor number

Notice that you didn’t see the error on the screen because bash

sent the error to the file instead.

Next, let’s use the standard input stream. Run the following

command in the shell to supply the contents of output.txt as input to

the cat command:

$ cat < output.txt

Hello World!

Goodbye!

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Redirecting standard input to the cat command

What if we wanted to redirect multiple lines to a command? Here

document redirection (<<) can help with this:

$ cat << EOF

Black Hat Bash

by No Starch Press

EOF

Black Hat Bash

by No Starch Press

Listing 1-15

A here document

In this example, we pass multiple lines as input to a command.

The EOF in this example acts as a delimiter marking the start and

end points of the input. Here document redirection treats the input as

if it were a separate file, preserving line breaks and whitespace.

The pipe operator (|) redirects the output of one command and

uses it as the input of another. For example, we could run the ls

command on the root directory and then use another command to

extract some data from it, as shown here:

$ ls -l / | grep "bin"

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