Скачать или смотреть Capturing Command Output in Shell Script Variables: A Guide to Fixing Common Issues

Learn how to successfully capture command output in shell script variables. This guide provides a clear solution to the common problem of variables not capturing command results, with step-by-step guidance.
---
This video is based on the question https://stackoverflow.com/q/62471085/ asked by the user 'Rohit Verma' ( https://stackoverflow.com/u/12759990/ ) and on the answer https://stackoverflow.com/a/62474592/ provided by the user 'Petryanu' ( https://stackoverflow.com/u/3001115/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Count is not Captured in Shell Script Variable

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Capturing Command Output in Shell Script Variables: A Guide to Fixing Common Issues

If you're working with shell scripts, you may encounter situations where you're trying to capture the output of a command into a variable, but nothing gets stored. This can be a frustrating problem, especially when you need to employ that value later in your script. One common example of this issue involves checking the count of active processes. In this guide, we'll discuss a specific case where count is not captured correctly in a shell variable and provide a straightforward solution to fix it.

The Problem

Suppose you're using a command to check for active processes related to a script named BDD_PreCheck.sh. You might write something like this in your shell script:

[[See Video to Reveal this Text or Code Snippet]]

However, you notice that count_build_status is null despite the command running correctly when executed directly in the terminal. The goal is to capture the output of the command to determine whether there are active processes related to BDD_PreCheck.sh.

Diagnosing the Issue

The problem arises from how the command substitution and output redirection are structured. When you use backticks to retrieve command output, any subsequent piping (like using | tee) can interrupt the assignment to the variable. This results in the variable being empty instead of capturing the expected output.

The Solution

To resolve this issue, you'll want to adjust your command so that tee is executed within the command substitution. This way, the output is captured correctly into the variable while still appending to the log file. Here’s the corrected version of your code:

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Made:

Included tee in the command substitution: By appending | tee -a ${logFile} within the backticks, you ensure that the output of the entire command, including the logging action, is captured in count_build_status.

Conclusion

In conclusion, handling output in shell scripts can be tricky, especially when involving command substitution and redirection. By ensuring that all required operations are included within the backticks, you can successfully store the output of commands in variables for further use in your scripts. Implementing this small adjustment can save you time and reduce frustration in debugging shell scripts. Happy scripting!