I have a situation where I output results from a CLI program:
e.g. input.exe > output.txt
Although output.txt already contains text, by default it gets overwritten when using the command above, how can you write the output to output.txt without overwriting existing data?
Use the >> operator; it appends the output to the existing content of the file.
input.exe >> output.txt
Related
My understanding is that the redirection operator, <, should allow me to take text from a file and give it as input to another file as if I had written out the contents of that file. Here is what I am trying to do:
python code.py < input.txt
I expect this to act as though I had typed the contents of input.txt after python code.py, but instead it acts as if I passed no input.
If I use cat, I get the contents of the file:
> cat input.txt
['2015-1-1','2015-5-1','2015-9-1','2015-10-1','2015-12-1','2016-1-1','2016-2-1','2016-4-1','2016-5-1'] [65,50,30,45,55,39,45,30,20]
And if I just copy and paste the contents of the file, I get the correct behavior.
I know this must be a really simple misunderstanding on my part, but I can't figure it out.
It's called Redirection, not piping, but you are correct that the < operator will push the file to the command. You can see this in action by using Sort instead of echo.
sort < input.txt
This will display the text file as a list, sorted alphabetically. Echo does not work with text files, so sending a text file to Echo simply runs "Echo".
If you just want to send a file to the command window, you can use Type instead, and not use the redirector.
type input.txt
I'm writing commands that do something like ./script > output.txt so that I can use the files in later scripts like ./script2 output.txt otherFile.txt > output2.txt. I remove them all at the end of the script, but when I'm testing certain things or debugging it's tricky to search through all my sub directories and files which have been created in the script.
Is the best option just to create a hidden file?
As always, there are numerous ways to do so. If you want to avoid files altogether, you can save the output (STDOUT) of a command in a variable and pass it to the next command as a file using the <() operator:
output=$(cat /usr/include/stdio.h)
cat <(echo "$output")
Alternatively, you can do so in a single command line:
cat <(cat /usr/include/stdio.h)
This assumes that the next command strictly requires a file for input.
I tend to avoid temporary files whenever possible to eliminate the need for a cleanup step that gets executed in all cases unless large amounts of data have to be processed.
The following command line executes properly:
MagBoltz32 < input.txt > out.txt
The executable requires the input in brackets as above and outputs the text file. The following "batch.BAT" file (now including the escape characters) does NOT execute but simply hangs as if expecting more input.
MagBoltz32 ^< input.txt ^> out.txt
Executing batch.BAT simply results in a blinking cursor. Ultimately this command line will go into FOR loop which loops over several input/output files. I do have access to the source code.
That's not "input in brackets". The programm doesn't use any parameters (at least not in the shown syntax). It gets it's input from STDIN and writes to STDOUT.
<input.txt redirects the content of input.txt to STDIN, so the program is able to use it, like it were entered per keyboard.
>output.txt redirects STDOUT to the file output.txt instead of writing it to the screen.
Escaping characters is only needed, if you want the special char to be shown on the screen instead of "executing" it (simply spoken), so in your case, escaping the redirection characters makes no sense, but disables the redirection.
In some OS's you need to escape the pipes <, > and | so the syntax could be completely valid. This is particularly true for batch files (http://www.robvanderwoude.com/redirection.php) ... Have you checked to make sure all the files are in the same path that is executing the batch file? If you run the batch from C:\foo even though bath.BAT is in c:\bar, the input.txt will be expected to be in C:\foo.
I have an exe and I am redirecting the output as follows.
mastercore.exe > D:\\Output.txt
or
mastercore.exe dir > D:\\Output.txt
The problem is the output.txt is recreated each time I start the command. I want the output.txt to be appended, rather. Can someone please suggest how this can be done?
use double >>:
mastercore.exe >> D:\\Output.txt
Very simple task: grab an output and redirect it to a file, using shell.
Now, I need to create the file if it does not exist, or if it exist, I need to append data
If I use output > file.txt I get data overwritten at each access. Not what I want.
If I use output >> file.txt I get an error, because shell is trying to access the file while, I need it to create it first (it does not exist yet).
Is there a way to say in one line to create it, if the file doesn't exist, and append data or append only if the file does exist already?
I can just do that using an if-else (if file exist use >> otherwise use >), and I would like to avoid to complicate my code, if there is an easier way.
to create or append to a file in bash, you can use >>.
echo "sample text" >> sample_file
echo "append text" >> sample_file