Is it possible to build a loop of commands with pipe or something else

command linepipe

So far, I know the piping mechanism as a way to connect a series of commands by connecting the stdout of one command to the stdin of the next command until the last command is reached, which connects its stdout with the display or a file.

Would it be possible, however, to make a loop out of commands, so the last command's stdout connects to the first command's stdin and maybe by using tee somehow the changing values of a certain output could be displayed?

Best Answer

Well you certainly could by just make a loop and use variables:

while true; do
    a=$(echo "$a" | grep "Hey" | cut -d" " -f2 | tee -a log)
done

That would save the last output which would be used at the beginning again

Related Solutions

What is Meant by Connecting STDOUT and STDIN?

Standard input and standard output are not commands.

Imagine commands as machines in a factory with an assembly line. Most machines are designed to have one conveyor belt to feed data in and one conveyor belt to feed data out; they are the standard input and the standard output respectively. The standard error is an opening on the side of the machine where it can eject rejects.

+-------+     +------------------+       +------------------+     +------+
| input |     |    machine A     |       |    machine B     |     |output|
| reser |=====|<stdin     stdout>|=======|<stdin     stdout>|=====|bucket|
| ‑voir |  →  |      stderr      |   →   |      stderr      |  →  |      |
+-------+     +------------------+       +------------------+     +------+
                      ||                          ||

The diagram above shows a conveyor belt that goes through two machines. The data comes from the input reservoir on the left, is fed to machine A, then the output is conveyed further to machine B (for which it is input), and machine B's output is deposited in the output bucket on the right.

In unix terms, this is called a pipeline. The metaphor is that of plumbing: a pipe connects machine A to machine B. The shell syntax for the pipeline above is

<input-file.txt commandA | commandB >output-file.txt

The < redirection symbol tells the shell to connect commandA's standard input to the file input-file.txt before launching commandA. (You can put the redirection before or after the command name.) The > redirection symbol tells the shell to connect commandB's standard output to output-file.txt. The pipe ("|") symbol in the middle tells the shell to connect commandA's standard output to commandB's standard input before launching them.

Commands can have more than one input and more than one output, but that's material for another day.

Shell – How stable are Unix shell “stdin/stdout APIs”

The POSIX 2008 standard has a section describing "Shell and Utilities". Generally, if you stick to that your scripts should be fairly future-proof, except possibly for deprecations, but those hardly happen overnight so you should have plenty of time to update your scripts.

In some cases where output format for a single utility varies widely across platforms and versions, the POSIX standard may include an option typically called -p or -P which specifies a guaranteed and predictable output format. An example of this is the time utility, which has widely varying implementations. If you need a stable API/output format, you would use time -p.

If you need to use a filter utility that is not covered by the POSIX standard, then you are pretty much at the mercy of the distribution packagers / upstream developers, just as you are at the mercy of the remote web developers when doing web scraping.

Best Answer

Related Solutions

What is Meant by Connecting STDOUT and STDIN?

Shell – How stable are Unix shell “stdin/stdout APIs”

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