Save Output in a Variable with $( ) (Linux Essentials)

You know how to save output in a file with >. To save output in a variable, use $( ):

text

learner@tidepool:~$ ERRORS=$(grep -c ERROR ~/logs/till.log)
learner@tidepool:~$ echo "errors today: $ERRORS"
errors today: 2

ERRORS=$(grep -c ERROR till.log)you type

ERRORS=2grep writes 2

The shell runs the inside first, then puts its output in the line

This is the same idea as variables and wildcards: the shell changes the line first, then runs it.

Use it anywhere in a line

$( ) works in any argument, also inside double quotes:

text

learner@tidepool:~$ echo "report made by $(whoami) on $(date +%A)"
report made by learner on Monday

date +%A writes the name of the day. +%A tells date what to write. date --help lists the other codes.

Use wc -l < FILE, so that only the number goes into the variable, with no file name:

text

learner@tidepool:~$ COUNT=$(wc -l < ~/orders.txt)
learner@tidepool:~$ echo "orders: $COUNT"
orders: 12