Checking if STDIN has been Redirected in a Shell Script

In shell scripting, input can be read from the keyboard (standard input, STDIN) or from a file (if the input is redirected). This article will guide you on how to check if STDIN has been redirected in a shell script.

1. **The Test Command**

The `test` command is a built-in shell command used to test if a condition is true or false. In this case, we'll use the `-t` option to test if STDIN is associated with a file descriptor, which indicates that STDIN has been redirected.

Here is a simple script demonstrating the use of the `test` command:

```bash
#!/bin/bash

if [ -t 0 ]; then
echo "STDIN is connected to a terminal"
else
echo how to check if std in has be redirected is redirected"
fi
```

In the script above, `0` represents STDIN.

If STDIN is connected to a terminal, the script will output "STDIN is connected to a terminal". If STDIN has been redirected, it will output "STDIN is redirected".

2. **The Read Command**

Another way to check if STDIN has been redirected is by using the `read` command.

When input is redirected, the `read` command returns a status of 1. If input is not redirected, it returns a status of 0.

Here's an example script using the `read` command:

```bash
#!/bin/bash

if (( $?(read -r _) )); then
echo "STDIN is redirected"
else
echo "STDIN is connected to a terminal"
fi
```

In this script, `read -r _` reads a line from STDIN into the `_` variable without interpreting backslashes, and the `$?` variable stores the exit status of the last command executed.

If the exit status is non-zero (indicating that STDIN has been redirected), the script will output "STDIN is redirected".

3. **Combining the Methods**

For more robust checking, you can combine the two methods:

```bash
#!/bin/bash

if [ -t 0 ] && (( $?(read -r _) )); then
echo "STDIN is connected to how to check if std in has be redirected terminal and redirected"
fi

if [ -n "$0" ]; then
echo "STDIN is redirected from a file named $0"
fi
```

In this script, the first condition checks if STDIN is connected to a terminal and if it is redirected.

The second condition checks if STDIN is redirected from a file named `$0`, which is a special variable representing the name of the script.

In conclusion, checking if STDIN has been redirected in a shell script can be achieved using the `test` command with the `-t` option, the `read` command, or a combination of both methods.

This knowledge can help you write more versatile shell scripts that can handle various input scenarios.