Shell Scripting – Standard Input, Output and Error

How can I redirect the output of a shell script to a file?

You can redirect the output of a shell script to a file using the `>` operator.

For example

./myscript.sh > output.txt

This will redirect the standard output to a file named ‘output.txt’.

What is the purpose of 2>&1 in shell scripting?

The expression `2>&1` is used to redirect the standard error (file descriptor 2) to the same location as the standard output. It combines both standard output and standard error streams, allowing them to be redirected together.

How do I read input from a user within a shell script?

You can use the `read` command to prompt the user for input within a shell script.

For example

read -p "Enter your name: " username

This will prompt the user for their name and store the input in the variable ‘username‘.

What is the key difference between >& and > in shell redirection?

The` >` operator is used for redirecting standard output, while `>&` is used to redirect both standard output and standard error.

For example

command > output.txt

It redirects standard output to a file.

command >& log.txt

It redirects both output and error to a log file.

How can I suppress both standard output and standard error in a shell script?

You can use the `>/dev/null 2>&1` redirection to discard both standard output and standard error.

For instance

command >/dev/null 2>&1

It will run ‘command’ without displaying any output or error messages.

Working on Linux applications we have several ways to get information from outside and to put it inside: command line args, environment variables, files. All of these sources are legal and good. But it has a finite size. Another way to establish communication is standard streams: input stream (stdin) used for getting data from outside of the app, output stream (stdout) to put data outside of the app, and error to put data outside of the app (stderr).

Each stream acts like a pipe: It has the same buffer to write and read the data. This buffer is available for reading from one application and available for writing from another one. On reading, the occupied buffer size will be reduced and it will be increased on writing. If the average rate of reading and writing is equal – then data passed over the stream can be any number of bytes long.