If you pipe into shell commands, that creates a subshell, i.e.
better written as
The general idea is to use as few pipes and use as few external programs as possible. Someone might write a slow-performing loop like this:
which creates four processes per loop just to do something read can do as a builtin:
In summary, pipes create extra processes or subshells, external programs create extra processes, and grouping ( ) create subshells, i.e. ( echo foo ; echo bar ) | myprogram
Hi all,
I have a shell script which calls other shell scripts, depending on the input.
Within a.sh, I have a command which calls b.sh (ie. ksh b.sh)
Normally, we use the exit function to terminate a shell. However, if I choose to call exit from b.sh, I will return to the parent shell who... (4 Replies)
hi,
i have to type certain commands again and again eg:
ps -ef | grep xyz
is there any way to replace the whole command by a short form
like i just do :
$ psl
and it give me the same output as
$ps -ef | grep xyz
where can we store these commands .....
on unix system and create... (2 Replies)
I have the following script running with nohup on one of my servers:
#!/bin/bash
#\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
#set log number
#i=1
#\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
#Check if log exits, if so incrememnt log number up so we don't clobber
#while... (8 Replies)
I've written a program in C, called count_0.1 which is essentially a word count program.
I want to be able to use it as a command in the terminal (by typing in count), like when you type in ls, you don't have to go to a directory, find an executable and type in: ./ls
I've tried:
Adding... (1 Reply)
Use and complete the template provided. The entire template must be completed. If you don't, your post may be deleted!
1. The problem statement, all variables and given/known data:
Here is what I am supposed to do, word for word from my assignment page:
1. Create/modify and print a... (2 Replies)
Hello,
I have a database of name variants with the following structure:
variant=variant=variant
The number of variants can be as many as thirty to forty.
Since the database is quite large (at present around 60,000 lines) duplicate sets of variants creep in. Thus
John=Johann=Jon
and... (2 Replies)
Can someone post a "for dummies" explanation on how subshells work?
Let's say I want to get a couple of pieces of info from the /etc/passwd file and set them as variables to be echo'd back after all data is gathered. something like
for i in ${grep 5000 /etc/passwd | cut -d : -f 5}... (5 Replies)
Hi,
I am trying to make a Shell Script using which I will update around 10K records in MySQL db each night. For that, I am currently doing the following command in FOR LOOP:
mysql -D $MASTER_DB_NAME -h $MASTER_DB_HOST -u $MASTER_DB_USER -p$MASTER_DB_PASSWD -e "$SQL_Query"
This command is... (2 Replies)
Hi there,
Everyone knows that errexit is NOT inherited by subshells. But does anyone know why errexit still appears as set with set +o?
The following example highlights the fact that errexit appears set in a subshell (set +o) even though it's actually unset (it does NOT exit with false).
$... (0 Replies)
So i have a script that is like this:
#!/bin/sh
VARA="elementary 1
elementary 2
nursery A
nursery B
highschool AZ"
echo "${ContentOfADifferentSCRIPT}" | sh
i do not have control over the script that is inside the variable "${ContentOfADifferentSCRIPT}".
however, i know that the... (3 Replies)
Discussion started by: SkySmart
3 Replies
LEARN ABOUT ULTRIX
pipe
pipe(2) System Calls Manual pipe(2)Name
pipe - create an interprocess channel
Syntax
include <limits.h> /*Definition of PIPE_MAX*/
pipe(fildes)
int fildes[2];
Arguments
fildes Passing an address as an array of two integers into the system call.
Description
The system call creates an I/O mechanism called a pipe. The file descriptors returned can be used in and operations. Their integer values
will be the two lowest available at the time of the function call. The O_NONBLOCK and FD_CLOEXEC flags will be clear on both file descrip-
tors.
When the pipe is written using the descriptor fildes[1], up to PIPE_MAX bytes of data are buffered before the writing process is suspended.
A read using the descriptor fildes[0] picks up the data.
It is assumed that after the pipe has been set up, two (or more) cooperating processes (created by subsequent calls) pass data through the
pipe with and calls.
The shell has a syntax to set up a linear array of processes connected by pipes.
For further information on how and calls behave with pipes, see the and reference pages.
A signal is generated if a write on a pipe with only one end is attempted.
Restrictions
Should more than 4096 bytes be necessary in any pipe among a loop of processes, deadlock may occur.
The underlying implementation of pipes is no longer socket based, but rather implemented through the file system. Any application that
needs socket functionality from pipes should use the system call.
Return Values
The function value zero is returned if the pipe was created; -1 if an error occurred.
Diagnostics
The call fails if:
[EMFILE] Too many descriptors are active.
[ENFILE] The system file table is full.
[EFAULT] The fildes buffer is in an invalid area of the process's address space.
Environment
Differs from the System V definition in that ENFILE is not a possible error condition.
See Alsosh(1), fork(2), read(2), socketpair(2), write(2)pipe(2)