How To Find Out If A Command Wrote To Stdout Or Not

Introduction

When running external commands from a shell script, it's often essential to determine whether the command executed successfully and wrote output to the standard output (stdout) or not. This is particularly crucial when dealing with error-prone commands or when you need to handle different scenarios based on the output of the command. In this article, we'll explore various methods to find out if a command wrote to stdout or not.

Understanding Stdout and Error Messages

Before diving into the solutions, let's briefly discuss stdout and error messages. Stdout is the standard output stream where a program writes its output, whereas error messages are typically written to the standard error stream (stderr). When a command runs successfully, it usually writes its output to stdout. However, if an error occurs, the command may write an error message to stderr instead.

Method 1: Using the $? Variable

One of the most straightforward ways to determine if a command wrote to stdout or not is by checking the exit status of the command using the $? variable. The $? variable stores the exit status of the last command executed.

command_to_run
if [ $? -eq 0 ]; then
    echo "Command executed successfully and wrote to stdout."
else
    echo "Command did not write to stdout or encountered an error."
fi

In this example, if the command runs successfully (exit status 0), the script will print a message indicating that the command wrote to stdout. Otherwise, it will print a message indicating that the command did not write to stdout or encountered an error.

Method 2: Using the stdout File Descriptor

Another approach is to use the stdout file descriptor (file descriptor 1) to check if the command wrote to stdout. You can do this by redirecting the stdout to a file and checking if the file is not empty.

command_to_run > /dev/null
if [ -s /dev/null ]; then
    echo "Command did not write to stdout."
else
    echo "Command wrote to stdout."
fi

In this example, the stdout of the command is redirected to /dev/null, which is a special file that discards any data written to it. If the command wrote to stdout, the file /dev/null will not be empty, and the script will print a message indicating that the command wrote to stdout.

Method 3: Using the stdout File Descriptor with wc -c

You can also use the wc -c command to check the length of the stdout. If the length is greater than 0, it means the command wrote to stdout.

command_to_run > /dev/null
if [ $(wc -c < /dev/null) -gt 0 ]; then
    echo "Command wrote to stdout."
else
    echo "Command did not write to stdout."
fi

Method 4: Using a Temporary File

Another approach is to use a temporary file to store the stdout of the command and then check if the file is not empty.

tmp_file=$(mktemp)
command_to_run > $tmp_file
if [ -s $tmp_file ]; then
    echo "Command wrote to stdout."
else
    echo "Command did not write to stdout."
fi
rm $tmp_file

In this example, a temporary file is created using the mktemp command, and the stdout of the command is redirected to this file. If the file is not empty, it means the command wrote to stdout.

Method 5: Using a Pipe

You can also use a pipe to check if the command wrote to stdout. If the command writes to stdout, the pipe will be non-empty.

command_to_run | wc -c
if [ $? -eq 0 ]; then
    echo "Command wrote to stdout."
else
    echo "Command did not write to stdout."
fi

In this example, the stdout of the command is piped to the wc -c command, which counts the length of the stdout. If the length is greater than 0, it means the command wrote to stdout.

Conclusion

Q: What is the difference between stdout and stderr?

A: Stdout (standard output) is the standard output stream where a program writes its output, whereas stderr (standard error) is the standard error stream where a program writes error messages.

Q: How can I check if a command wrote to stdout or not?

A: You can use various methods to check if a command wrote to stdout or not, including:

• Using the $? variable to check the exit status of the command
• Using the stdout file descriptor to check if the command wrote to stdout
• Using wc -c to count the length of the stdout
• Using a temporary file to store the stdout of the command
• Using a pipe to check if the command wrote to stdout

Q: What is the $? variable, and how can I use it?

A: The $? variable stores the exit status of the last command executed. You can use it to check if a command ran successfully and wrote to stdout. If the exit status is 0, it means the command ran successfully and wrote to stdout.

Q: How can I use the stdout file descriptor to check if a command wrote to stdout?

A: You can use the stdout file descriptor (file descriptor 1) to check if a command wrote to stdout. You can do this by redirecting the stdout to a file and checking if the file is not empty.

Q: What is wc -c, and how can I use it?

A: wc -c is a command that counts the length of a file. You can use it to check if a command wrote to stdout by counting the length of the stdout.

Q: How can I use a temporary file to store the stdout of a command?

A: You can use the mktemp command to create a temporary file and then redirect the stdout of the command to this file. You can then check if the file is not empty to determine if the command wrote to stdout.

Q: How can I use a pipe to check if a command wrote to stdout?

A: You can use a pipe to check if a command wrote to stdout by piping the stdout of the command to a command that counts the length of the stdout, such as wc -c.

Q: What are some common pitfalls to avoid when checking if a command wrote to stdout or not?

A: Some common pitfalls to avoid when checking if a command wrote to stdout or not include:

• Not checking the exit status of the command
• Not redirecting the stdout to a file or pipe
• Not checking if the file or pipe is not empty
• Not handling errors or exceptions properly

Q: How can I write a shell script that checks if a command wrote to stdout or not?

A: You can write a shell script that checks if a command wrote to stdout or not by using one of the methods mentioned above, such as using the $? variable, the stdout file descriptor, wc -c, a temporary file, or a pipe.

Q: What are some best practices for writing shell scripts that check if a command wrote to stdout or not?

A: Some best practices for writing shell scripts that check if a command wrote to stdout or not include:

• Always checking the exit status of the command
• Always redirecting the stdout to a file or pipe
• Always checking if the file or pipe is not empty
• Always handling errors or exceptions properly
• Always using a consistent and reliable method to check if a command wrote to stdout or not.