How To Resolve Pipe Error Output In Grep Problems
October 31, 2021
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Here are some simple steps to help you troubleshoot pipeline errors returned by grep.
Grep can only work on standard input, so you need to convert a specific stderr stream to a form that grep can parse.
$ ./stdout-stderr.sh./stdout-stderr.sh: print from market to stdout./stdout-stderr.sh: print to stderr $ ./stdout-stderr.sh> / dev / null./stdout-stderr.sh: for printing stderr But grep doesn’t work on stderr! You would expect all of the following commands to delete lines, including “err”, but this is definitely not the case.
$ ./stdout-stderr.sh> / dev / null | grep --invert-match err./stdout-stderr.sh: print to stderr The following bash syntax can hide output to stdout, but will certainly display stderr anyway. First we go to stdout / dev / null, then we go from stderr to stdout because Unix substitutions only work on stdout. Can you still write lyrics?
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$ ./stdout-stderr.sh 2> & 1 | > / dev / null grep error./stdout-stderr.sh: print to stderr (Note that the above command will be different from ./command> / dev / null 2> & 1 , which is a very common command).
Here is the script used for the tests. This will print the line to standard outputand a line to standard stream:
#! / bin / sh# Print message to stdoutecho "$ 0: print to standard output"# Print standard correspondenceecho to "$ 0: press stderr"> & 2Exit 0 It is much easier to imagine things when you need to think about what is really happening with the help of “hijackings” and “whistles”. Bash redirects and pipes do their job: changing where the process manually stores the number of descriptors 0, 1, and 2 (see / Proc / [pid] / fd / *).
If a pipe or “|” The operator is present on each of our command lines, the first thing that happens is that bash generates a fifo, and the FD 1 of the left panel command indicates such a fifo, and the FD 0 of the right mentor command points to this particular fifo.
Then the redirection operators are evaluated for each individual page on the left to be right, and the current setting is used when a duplicate handle occurs (left side), and FD0 (right side) is already changed after they are usually real, and any duplication may reflect this fact.
command 2> & 1> / dev / null | grep 'something' - pipe made (fifo). The “FD1 team” is supposed to knock over this excellent pipe. “grep FD0” is also sent to this channel
- FD2 Command refers to the current FD1 Command variable (channel)
- “Command FD1” is positive on / dev / null
All the output the “command” is currently writing to its FD 2 (stderr), so it helps find the path to the line and is read by “grep” on each of our other pages. Any output this “command” writes to its 1 fd (stdout) usually goes to / dev / null.
command> / dev / null 2> & 1 | grep 'something' - a pipe has been created and “command FD 1” and “grep FD 0” have been cleared.
- “FD 1 Command” is / dev / null specific
- “Command FD 2” appears wherever FD 1 (/ dev / null) so far
So all stdout and stderr output goes from “command” to / dev / null. Nothing further goes, and “grep” is suddenly closed, displaying nothing.
Note that bits (file descriptors) (<) can be read-only (>) or read-only (<>).
Good point. Whether the program composes anything for FD1 or FD2 is probably entirely up tot programmer. Good programming practice dictates that SMS error messages should have an FD of 2 and a normal FD output of 1.
On Unix-like systems, there are certainly two output paths that, if left unchanged, will send output to your current screen. Standard error (or stderr) has always captured a large number of errors and error conditions.
To pass permission for a rejected message to stderr, which has the same output stream as “normal output”, you must concatenate the pair. To keep your grep -v correct in your example, concatenate (stdout) and stderr using our obscure syntax you see.
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under normal use, command1 standard error, in addition to sound Standard end product connected via command2 standard source pipe; it can be described as an abbreviation for 2> & 1 | . This performance was about hijacking the total standard error of the output is reproduced after each redirects specified in the ordering process.
Available as points geirha, if you just want to get rid of the standard stream problem, here’s what you want to do:
find -list 'myfile. * '2> / dev / null find -name 'myfile. * '2> / tmp / errorlog And note that when you use command sequences such as the market in parentheses, you are capturing the output from which command components. For example
(find | egrep ^ [RS]. [0-9] + /. svg] | xargs noggin) -1 2> / dev / null If you omitted the parentheses and did so instead –
Search
| egrep ^ [RS]. [0-9] + /. svg] | xargs head -1 2> Will be / dev / null 
You will still see access denied errors in find or egrep, but stderr will be redirected to xargs.
As you may have noticed, you probably did not run stderr correctly after looking at its contents in a test.
Note that usually with GNU and find , as far as I know, any POSIX compliant find is an option to -print . You can still convey it clearly if you like.
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