Working with shell scripts has always been interesting for programmers and sysadmins because the output helps both of them with debugging and monitoring. The configuration of most Linux distributions is largely based on files, so it is important to understand the concept of writing data to a text file using a script or redirecting output at the command line.
Linux uses three main data streams while communicating to the user and the computer:
- stdin (STandarD INput)
- stdout (STandarD OUTput)
- stderr (STandarD ERRor)
This is the data stream for the input of information. Any input from any device such as a keyboard or a mouse comes under the standard input data stream. stdin is represented by 0 Stream ID.
[ You might also enjoy reading: Five ways to use redirect operators in bash ]
This is the data stream for the output of data. The output from devices (like monitor, speaker, etc.) comes under the standard output data stream. stdout is represented by 1 Stream ID.
Standard error is used to handle any errors produced by the commands. Any device stream (like monitor, speaker, etc.) that warns the user that something has gone wrong comes under stderr. stderr is represented by 2 Stream ID.
How do you write data to a file?
Use redirection operators to fetch the data from the stdout and stderr streams and redirect them to a text file.
Redirection: Redirection is a Linux feature used to change input/output devices while executing a command.
To write data to a text file from a Bash script, use output/error redirection with the
>> redirection operators.
> Overwrites data in a text file.
>> Appends data to a text file.
Creating a basic script and understanding the redirection
date >> test1.txt who >> test1.txt date > test2.txt who > test2.txt
Here the output of both commands will be appended to
test2.txt will contain only the output of the
Working further with redirection
The above script will redirect only the output and will fail in case of error. To work with redirection for output, error, or both, you must specify the extra parameters.
For redirecting output:
For redirecting error:
For redirecting both:
date --who 2>> test1.txt who 2>> test1.txt
For redirecting only the errors, you've specified the specific parameter for the error. The output file will contain only the output of the first command because it has a wrong argument.
Other arguments can be used interchangeably to achieve different functionality.
Writing the script for other commands is the same as the above script and requires the operators displayed above.
[ Download now: A sysadmin's guide to Bash scripting. ]
Redirecting data to stdout or stderr is very useful to developers and sysadmins alike. Understanding these tools and their results will help you create new files, troubleshoot, and gather system information.