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Top Forums UNIX for Beginners Questions & Answers One Line Command how to use pipe statements to execute and comment on multiple possible outcomes Post 303040995 by greavette on Monday 11th of November 2019 10:09:20 PM
Old 11-11-2019
One Line Command how to use pipe statements to execute and comment on multiple possible outcomes

Hello Forum,

I'm looking to expand the following command:

Code:
INACTIVE_KERNELS=$(python -mplatform | grep -qi red && rpm -qa | grep '^kernel-[0-9]' |grep -vE `uname -r` | paste -sd \; || echo "Not Red Hat Server")

Currently this command will check if my server is RedHat server using the grep -qi red statement. If my Server is not RedHat I echo an statement that says it's not a red hat server.

I've just discovered that if the server is Red Hat and this
Code:
rpm -qa | grep '^kernel-[0-9]' |grep -vE `uname -r` | paste -sd \;

results with no inactive kernels, nothing gets written out to this INACTIVE_KERNELS variable. I would like to have my one line code to either write out the Inactive Kernels or to write out a message that says "No Inactive Kernels Found".
 

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PIPE(2) 							System Calls Manual							   PIPE(2)

NAME
pipe - create an interprocess channel SYNOPSIS
#include <u.h> #include <libc.h> int pipe(int fd[2]) DESCRIPTION
Pipe creates a buffered channel for interprocess I/O communication. Two file descriptors are returned in fd. Data written to fd[1] is available for reading from fd[0] and data written to fd[0] is available for reading from fd[1]. After the pipe has been established, cooperating processes created by subsequent fork(2) calls may pass data through the pipe with read and write calls. The bytes placed on a pipe by one write are contiguous even if many processes are writing. Write boundaries are preserved: each read terminates when the read buffer is full or after reading the last byte of a write, whichever comes first. The number of bytes available to a read(2) is reported in the Length field returned by fstat or dirfstat on a pipe (see stat(2)). When all the data has been read from a pipe and the writer has closed the pipe or exited, read(2) will return 0 bytes. Writes to a pipe with no reader will generate a note sys: write on closed pipe. SOURCE
/sys/src/libc/9syscall SEE ALSO
intro(2), read(2), pipe(3) DIAGNOSTICS
Sets errstr. BUGS
If a read or a write of a pipe is interrupted, some unknown number of bytes may have been transferred. When a read from a pipe returns 0 bytes, it usually means end of file but is indistinguishable from reading the result of an explicit write of zero bytes. PIPE(2)
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