Pass the output of previous command to next as an argument

argumentscommand linecommand-substitutionoutputpipe

I've a command that outputs data to stdout (command1 -p=aaa -v=bbb -i=4).
The output line can have the following value:

rate (10%) - name: value - 10Kbps

I want to grep that output in order to store that 'rate' (I guess pipe will be useful here).
And finally, I would like that rate to be a value of a parameter on a second command (let's say command2 -t=${rate})

It looks to be tricky on my side; I would like to know better how to use pipes, grep, sed and so on.

I've tried lots of combinations like that one but I'm getting confused of these:

$ command1 -p=aaa -v=bbb -i=4 | grep "rate" 2>&1 command2 -t="rate was "${rate}

Best Answer

You are confusing two very different types of inputs.

Standard input (stdin)
Command line arguments

These are different, and are useful for different purposes. Some commands can take input in both ways, but they typically use them differently. Take for example the wc command:

Passing input by stdin:
```
ls | wc -l
```
This will count the lines in the output of ls
Passing input by command line arguments:
```
wc -l $(ls)
```
This will count lines in the list of files printed by ls

Completely different things.

To answer your question, it sounds like you want to capture the rate from the output of the first command, and then use the rate as a command line argument for the second command. Here's one way to do that:

rate=$(command1 | sed -ne 's/^rate..\([0-9]*\)%.*/\1/p')
command2 -t "rate was $rate"

Explanation of the sed:

The s/pattern/replacement/ command is to replace some pattern
The pattern means: the line must start with "rate" (^rate) followed by any two character (..), followed by 0 or more digits, followed by a %, followed by the rest of the text (.*)
\1 in the replacement means the content of the first expression captured within $...$, so in this case the digits before the % sign
The -n flag of the sed command means to not print lines by default. The p at the end of the s/// command means to print the line if there was a replacement. In short, the command will print something only if there was a match.

Related Solutions

Bash – Multicolored Grep

Here's a different approach. I have a little Perl script which I have already posted in another answer that will highlight the user provided patterns in different colors. A slightly modified version of the script will act like grep:

#!/usr/bin/env perl
use Getopt::Std;
use strict;
use Term::ANSIColor; 

my %opts;
getopts('hic:l:',\%opts);
    if ($opts{h}){
      print<<EoF; 
Use -l to specify the pattern(s) to highlight. To specify more than one 
pattern use commas. 

-l : A Perl regular expression to be colored. Multiple expressions can be
     passed as comma separated values: -l foo,bar,baz
-i : makes the search case sensitive
-c : comma separated list of colors;

EoF
      exit(0);
    }

my $case_sensitive=$opts{i}||undef;
my @color=('bold red','bold blue', 'bold yellow', 'bold green', 
       'bold magenta', 'bold cyan', 'yellow on_blue', 
       'bright_white on_yellow', 'bright_yellow on_red', 'white on_black');
if ($opts{c}) {
   @color=split(/,/,$opts{c});
}
my @patterns;
if($opts{l}){
     @patterns=split(/,/,$opts{l});
}
else{
    $patterns[0]='\*';
}

# Setting $| to non-zero forces a flush right away and after 
# every write or print on the currently selected output channel. 
$|=1;

while (my $line=<>) 
{ 
    my $want=0;
    for (my $c=0; $c<=$#patterns; $c++){
    if($case_sensitive){
        if($line=~/$patterns[$c]/){
           $line=~s/($patterns[$c])/color("$color[$c]").$1.color("reset")/ge;
           $want++;
        }
    }
    else{
        if($line=~/$patterns[$c]/i){
          $line=~s/($patterns[$c])/color("$color[$c]").$1.color("reset")/ige;
          $want++;
        }
      }
    }
print STDOUT $line if $want>0;
}

If you save that script as cgrep somewhere in your PATH and make it executable, you can specify up to 10 different patterns, each of which will be printed in a different color:

enter image description here

$ cgrep -h
Use -l to specify the pattern(s) to highlight. To specify more than one 
pattern use commas. 

-l : A Perl regular expression to be colored. Multiple expressions can be
     passed as comma separated values: -l foo,bar,baz
-i : makes the search case sensitive
-c : comma separated list of colors;

Shell – Use piped stdin as argument to next command

Assuming that pgrep may return multiple PIDs:

$ pgrep mycommand | xargs -n 1 lsof -p

This will, for each PID, run lsof -p with the PID appended.

Best Answer

Related Solutions

Bash – Multicolored Grep

Shell – Use piped stdin as argument to next command

Related Question