get output and return value of grep in single operation in bash

Question

I am writing a bash script; I execute a certain command and grep.

pfiles $1 2> /dev/null | grep name # $1 Process Id

The response will be something like:

sockname: AF_INET6 ::ffff:10.10.50.28  port: 22
peername: AF_INET6 ::ffff:10.16.6.150  port: 12295

The response can be no lines, 1 line, or 2 lines.
In case grep returns no lines (grep return code 1), I abort the script; if I get 1 line I invoke A() or B() if more than 1 line. grep's return code is 0 when the output is 1-2 lines.

grep has return value (0 or 1) and output.
How can I catch them both ? If I do something like:

OUTPUT=$(pfiles $1 2> /dev/null | grep peername)

Then variable OUTPUT will have the output (string); I also want the boolean value of grep execution.

Answer 1

You can use

output=$(grep -c 'name' inputfile)

The variable output will then contain the number 0, 1, or 2. Then you can use an if statement to execute different things.

Answer 2

This is fairly simple:

OUTPUT=$(pfiles "$1" 2> /dev/null | grep peername)
grep_return_code=$?

If a $(…) command substitution is assigned to a variable, $? will get the return code from the last command in the $(…).  And, of course, you don’t need to refer to $? explicitly; you can do things like

if OUTPUT=$(pfiles "$1" 2> /dev/null | grep peername)
then
    # the rest of the script
                ︙
fi

or

if ! OUTPUT=$(pfiles "$1" 2> /dev/null | grep peername)
then
    exit
fi
# the rest of the script
            ︙

This approach is useful in situations where the output of the command and its return code (a.k.a. exit status) are uncorrelated.  But, for grep, they are highly correlated: If it produced output, it succeeded.  If it didn’t produce output, it failed.  So why not just test the output ?

OUTPUT=$(pfiles "$1" 2> /dev/null | grep peername)
if [ "$OUTPUT" ]
then
    # the rest of the script
                ︙
fi

or

OUTPUT=$(pfiles "$1" 2> /dev/null | grep peername)
if [-z "$OUTPUT" ]
then
    exit
fi
# the rest of the script
            ︙

P.S. You should always quote your shell variable references (e.g., "$1") unless you have a good reason not to, and you’re sure you know what you’re doing.

Answer 3

If you need the result of grep, you can not use the -c flag as outlined in the other answer. What you can do, though, is running twice, once with -c flag to get the number of matches and one without -c flag, to see the matches. However, this can be very inefficient, depending on the size of your input file.

You can do something like this:

Content of input:

The first line is foo
I have a drink at the bar
The third line is foo again

Content of script:

#!/usr/bin/env bash

countMatches(){
    echo Searching for "${1}"
    result=$(grep "${1}" input)
    if [ "$?" -ne 0 ]; then
        echo No match found
        echo
        exit 1
    fi

    if [ $(echo "${result}" | wc -l) -eq 1 ]; then
        echo 1 match found:
        echo "${result}"
        echo
    else
        echo 2 matches found:
        echo "${result}"
        echo
    fi
}

countMatches foo
countMatches bar
countMatches baz

And here's the output when you invoke the script:

Searching for foo
2 matches found:
The first line is foo
The third line is foo again

Searching for bar
1 match found:
I have a drink at the bar

Searching for baz
No match found

Answer 4

Try this code, I feel, it doing what you want.

1. I put grep output to the OUTPUT variable
2. I think, you don't need grep exit status as boolean value. It is 0 or 1, and it doesn't suit to your task. You need amount of lines - 0, 1, 2, etc. Therefore, count lines in the OUTPUT variable and put this number to the second variable - line_count. We will have two variables in the result. First with grep output, second with amount of lines.
3. Then, check line_count in the case statement and do needed action.

Usage: ./switching_by_grep_result.sh your_file.txt

#!/bin/bash

# your code
OUTPUT=$(pfiles $1 2> /dev/null | grep name) # $1 Process Id

# count lines in the OUTPUT variable by using 'grep -c'
line_count=$(echo -n "$OUTPUT" | grep -c '^')

# checks lines count in here and invokes needed script or exit.
# if 0 lines - exit
# if 1 lines - invoke A
# if any other number of lines - invoke B
case $line_count in  
    0) echo 'exit'
    ;;  
    1) echo 'A() script invoking here'
    ;;  
    *) echo 'B() script invoking here'
    ;;  
esac

Answer 5

I came across this whilst trying to figure out how to use grep as a test for some piped output, whilst still showing that output regardless. Note that I'm not grepping a file, but stdin.

My particular example is as follows:

./manage.py --dry-run makemigrations | grep "No changes detected"

The return code is 0 (success) when No changes detected. The return code is 1 (fail) when there are changes - but grep doesn't match and so the required changes are not printed!

My solution was as follows

./manage.py --dry-run makemigrations | tee /dev/stderr | grep -q "No changes detected"

This will always print the output, but will still return the correct error code.