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How to check if a command outputs an empty string?
Here’s an alternative approach that writes the std-out and std-err of some command a temporary file, and then checks to see if that file is empty. A benefit of this approach is that it captures both outputs, and does not use sub-shells or pipes.
How to check if a pipe is empty in Unix?
One easy way to check if there’s data available for reading in Unix is with the FIONREAD ioctl. I cannot think of any standard utility doing just that, so here is a trivial program doing it (better than the ifne from moreutils IMHO ;-)). fionread [ prog args ] If there’s no data available on stdin, it will exit with status 1.
What does the out null cmdlet do in win 10?
The Out-Null cmdlet sends its output to NULL, in effect, removing it from the pipeline and preventing the output to be displayed at the screen. This command gets items in the current location/directory, but its output is not passed through the pipeline nor displayed at the command line.
What does IFNE do if the input is empty?
If the input is empty, ifne does nothing and returns the status 0, which cannot be distinguished from the command running successfully. If you want to do something if the input is empty, you need to arrange for the command not to return 0, which can be done by having the success case return a distinguishable error status:
Do you need LS for command substitution in shell?
Note: The command substitution is done by the shell, not by ls, so you do not need ls. When you refer to a variable without quotes around it (e.g. echo $files ), the shell splits the value apart on whitespace and passes each term as a separate command-line option.
How can I use process substitution in Bash?
Like arrays, process substitution is a feature of bash and other advanced shells. It is not part of the POSIX standard. If desired, lastpipe can be used instead of process substitution (hat tip: Caesar ): shopt -s lastpipe tells bash to run the last command in the pipeline in the current shell (not the background).
What happens if a command outputs only newlines?
Therefore, if the command outputs only newlines, the substitution will capture nothing and the test will return false. While very unlikely, this is possible in the above example, since a single newline is a valid filename! More information in this answer.
What happens if a command does not return a string?
A command does not return a string (it returns a small integer exit code, usually 0 for success and 1 or 2 on failure), but it may output some data, to put in a string. – Basile Starynkevitch Aug 27 ’12 at 6:56
How to test for a non-empty string in Bash?
You can test for a non-empty string in Bash like this: Note that I’ve used -A rather than -a, since it omits the symbolic current (.) and parent ( ..) directory entries. Note: As pointed out in the comments, command substitution doesn’t capture trailing newlines.
How to check if a pipe is empty in Bash?
Unlike Standard/canonical way to test whether foregoing pipeline produced output? the input needs to be preserved to pass it to the program. This generalizes How to pipe output from one process to another but only execute if the first has output? which focuses on sending email.
How to know if a pipe has data?
The only way to know that a pipe has data is to read a byte, and then you have to get that byte to its destination.
Is there a way to check if output is always small?
Second, avoid unnecessary storing in RAM and processing of potentially huge data. The answer didn’t specify that the output is always small so a possibility of large output needs to be considered as well. This will run the command and check whether the returned output (string) has a zero length.