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Full Discussion: read command
Top Forums Shell Programming and Scripting read command Post 78784 by r2007 on Friday 22nd of July 2005 04:52:07 AM
Old 07-22-2005
FYI. From BASH Frequently-Asked Questions
Quote:
E4) If I pipe the output of a command into `read variable', why doesn't
the output show up in $variable when the read command finishes?

This has to do with the parent-child relationship between Unix
processes. It affects all commands run in pipelines, not just
simple calls to `read'. For example, piping a command's output
into a `while' loop that repeatedly calls `read' will result in
the same behavior.

Each element of a pipeline runs in a separate process, a child of
the shell running the pipeline. A subprocess cannot affect its
parent's environment. When the `read' command sets the variable
to the input, that variable is set only in the subshell, not the
parent shell. When the subshell exits, the value of the variable
is lost.

Many pipelines that end with `read variable' can be converted
into command substitutions, which will capture the output of
a specified command. The output can then be assigned to a
variable:

grep ^gnu /usr/lib/news/active | wc -l | read ngroup

can be converted into

ngroup=$(grep ^gnu /usr/lib/news/active | wc -l)

This does not, unfortunately, work to split the text among
multiple variables, as read does when given multiple variable
arguments. If you need to do this, you can either use the
command substitution above to read the output into a variable
and chop up the variable using the bash pattern removal
expansion operators or use some variant of the following
approach.

Say /usr/local/bin/ipaddr is the following shell script:

#! /bin/sh
host `hostname` | awk '/address/ {print $NF}'

Instead of using

/usr/local/bin/ipaddr | read A B C D

to break the local machine's IP address into separate octets, use

OIFS="$IFS"
IFS=.
set -- $(/usr/local/bin/ipaddr)
IFS="$OIFS"
A="$1" B="$2" C="$3" D="$4"

Beware, however, that this will change the shell's positional
parameters. If you need them, you should save them before doing
this.

This is the general approach -- in most cases you will not need to
set $IFS to a different value.

Some other user-supplied alternatives include:

read A B C D << HERE
$(IFS=.; echo $(/usr/local/bin/ipaddr))
HERE

and, where process substitution is available,

read A B C D < <(IFS=.; echo $(/usr/local/bin/ipaddr))
 

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scotty(1)							 Tnm Tcl Extension							 scotty(1)

__________________________________________________________________________________________________________________________________________________

NAME
scotty - A Tcl shell including the Tnm extensions. SYNOPSIS
scotty ?fileName arg arg ...? _________________________________________________________________ DESCRIPTION
scotty is a Tcl interpreter with extensions to obtain status and configuration information about TCP/IP networks. After startup, scotty evaluates the commands stored in .scottyrc and .tclshrc in the home directory of the user. SCRIPT FILES
If scotty is invoked with arguments then the first argument is the name of a script file and any additional arguments are made available to the script as variables (see below). Instead of reading commands from standard input scotty will read Tcl commands from the named file; scotty will exit when it reaches the end of the file. If you create a Tcl script in a file whose first line is #!/usr/local/bin/scotty2.1.11 then you can invoke the script file directly from your shell if you mark the file as executable. This assumes that scotty has been installed in the default location in /usr/local/bin; if it's installed somewhere else then you'll have to modify the above line to match. Many UNIX systems do not allow the #! line to exceed about 30 characters in length, so be sure that the scotty executable can be accessed with a short file name. An even better approach is to start your script files with the following three lines: #!/bin/sh # the next line restarts using scotty exec scotty2.1.11 "$0" "$@" This approach has three advantages over the approach in the previous paragraph. First, the location of the scotty binary doesn't have to be hard-wired into the script: it can be anywhere in your shell search path. Second, it gets around the 30-character file name limit in the previous approach. Third, this approach will work even if scotty is itself a shell script (this is done on some systems in order to handle multiple architectures or operating systems: the scotty script selects one of several binaries to run). The three lines cause both sh and scotty to process the script, but the exec is only executed by sh. sh processes the script first; it treats the second line as a comment and executes the third line. The exec statement cause the shell to stop processing and instead to start up scotty to reprocess the entire script. When scotty starts up, it treats all three lines as comments, since the backslash at the end of the second line causes the third line to be treated as part of the comment on the second line. VARIABLES
Scotty sets the following Tcl variables: argc Contains a count of the number of arg arguments (0 if none), not including the name of the script file. argv Contains a Tcl list whose elements are the arg arguments, in order, or an empty string if there are no arg arguments. argv0 Contains fileName if it was specified. Otherwise, contains the name by which scotty was invoked. tcl_interactive Contains 1 if scotty is running interactively (no fileName was specified and standard input is a terminal-like device), 0 otherwise. PROMPTS
When scotty is invoked interactively it normally prompts for each command with ``% ''. You can change the prompt by setting the variables tcl_prompt1 and tcl_prompt2. If variable tcl_prompt1 exists then it must consist of a Tcl script to output a prompt; instead of out- putting a prompt scotty will evaluate the script in tcl_prompt1. The variable tcl_prompt2 is used in a similar way when a newline is typed but the current command isn't yet complete; if tcl_prompt2 isn't set then no prompt is output for incomplete commands. SEE ALSO
Tnm(n), Tcl(n) AUTHORS
Juergen Schoenwaelder <schoenw@cs.utwente.nl> Tnm scotty(1)
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