Assigning the output of a command to a variable

Question

I am new with unix and I am writing a shell script.

When I run this line on the command prompt, it prints the total count of the number of processes which matches:

ps -ef | awk '/siebsvc –s siebsrvr/ && !/awk/ { a++ } END { print a }'

example, the output of the above line is 2 in the command prompt.

I want to write a shell script in which the output of the above line (2) is assigned to a variable, which will be later be used for comparison in an if statement.

I am looking for something like

output= `ps -ef | awk '/siebsvc –s siebsrvr/ && !/awk/ { a++ } END { print a }'`
echo $output

But when i run it, it says output could not be found whereas I am expecting 2. Please help.

Solution 1

You can use a $ sign like:

OUTPUT=$(expression)

Solution 2

Try:

output=$(ps -ef | awk '/siebsvc –s siebsrvr/ && !/awk/ { a++ } END { print a }'); echo $output

Wrapping your command in $( ) tells the shell to run that command, instead of attempting to set the command itself to the variable named "output". (Note that you could also use backticks `command`.)

I can highly recommend http://tldp.org/LDP/abs/html/commandsub.html to learn more about command substitution.

Also, as 1_CR correctly points out in a comment, the extra space between the equals sign and the assignment is causing it to fail. Here is a simple example on my machine of the behavior you are experiencing:

jed@MBP:~$ foo=$(ps -ef |head -1);echo $foo
UID PID PPID C STIME TTY TIME CMD

jed@MBP:~$ foo= $(ps -ef |head -1);echo $foo
-bash: UID: command not found
UID PID PPID C STIME TTY TIME CMD

Solution 3

If you want to do it with multiline/multiple command/s then you can do this:

output=$( bash <<EOF
#multiline/multiple command/s
EOF
)

Or:

output=$(
#multiline/multiple command/s
)

Example:

#!/bin/bash
output="$( bash <<EOF
echo first
echo second
echo third
EOF
)"
echo "$output"

Output:

first
second
third