Script output redirection problem

[johnmac]

Hi All,

Im strugging to write a script to take lines of output of one script and pass each line as a parameter to another script, i.e.

Run install script
For every line output by install script
Run status script with install script output line as the input

For example, if install generates the lines

"Done1"
"Done2"
"Done3"

I want to call

status "Done1"
status "Done2"
status "Done3"

Ive been trying stuff with pipes but to no avail.

Any help appreciated - thanks!

[drl]

Hi.

Suppose we have install as:

#!/bin/sh

echo "Done1"
echo "Done2"
echo "Done3"

exit 0

then we take each line from the output and do something with it -- here just an echo:

#!/bin/sh

# @(#) s1       Demonstrate processing output line by line.

set -o nounset
echo
echo "GNU bash $BASH_VERSION" >&2
echo

for i in $( ./install )
do
        echo " do something with line :$i:"
done

exit 0

To produce:

% ./s1

GNU bash 2.05b.0(1)-release

 do something with line :Done1:
 do something with line :Done2:
 do something with line :Done3:

See http://www.tldp.org/LDP/abs/html/index.html for help in bash scripting ... cheers, drl

[Cabhan]

drl's solution mostly works, but it's missing one key part. Before the for loop, you should set

IFS='\012'

The reason for this is that normally, the input to the for loop would get split on any whitespace. You only want it to split on newlines (octal 012, decimal 10). $IFS determines what the input splits on.

Now, drl's script works, but you would need to somehow tell the script the name of the program as a parameter or hardcode it in. That is one solution. A solution I prefer is:

#!/bin/bash

# @(#)  input_to_param          Demonstrates how to read STDIN and turn each line into a paramter to another program

# Usage: PROGRAM | input_to_param

while read line; do
        status "$line"
done

Now you simply execute a program, and pipe it into this script. For instance:

./install | ./input_to_param

As a final solution, I will share with you one of my favorite programs. It is called xargs. xargs is designed for situations where you want to do the same thing with multiple lines of input.

To use it is pretty simple:

./install | xargs -I{} status "{}"

Here, xargs will use '{}' to denote each line, and will call "status {}" on each line of input.

See how this massively simplifies the problem?

[drl]

Hi, Cabhan.

Yes, it's always good to get more than one point of view, and alternative methods of solving problems. Thanks for posting.

./install | xargs -I{} status "{}"

I copied this line, and substituted echo for status.

What is your version of xargs? Mine is "GNU xargs version 4.1.20", it prints {} literally, and it doesn't like -I{} ... cheers, drl

[Cabhan]

O.o

I'm using xargs 4.3.7.

alex@danu ~ $ xargs --version
GNU xargs version 4.3.7
Built using GNU gnulib version 2007-05-26
alex@danu ~ $ echo 'a b c' | xargs -I{} echo {}
a b c

Check your man page, but that is weird. I guess our versions are too far apart. Check for an option that lets you set replace-str.

As an interesting note, in this case, the "-I{}" is unnecessary, as by default, xargs simply appends each piece of input to the command. However, "-d \n" might be necessary, as by default xargs splits on whitespace, while we only want it to split on newlines. But since my "-d" was being weird (xargs was splitting on newlines anyway), I decided to throw it in just in case.