grep -oP '^MemFree: *K[0-9]+' /proc/meminfo

a=$(</proc/meminfo)
echo "$a" | grep -oP '^MemFree: *K[0-9]+'

a=$(</proc/meminfo)

printf '%sn' "$a" | grep '^MemFree'

grep '^MemFree' <<< "$a"

grep '^MemFree' << EOF
 $a
 EOF

IFS= read -rd '' meminfo < /proc/meminfo
memfree=${meminfo#*MemFree:}
memfree=${memfree%%$'n'*}
memfree=${memfree#"${memfree%%[! ]*}"}

memfree=${${"$(</proc/meminfo)"##*MemFree: #}%%$'n'*}

time for i in {1..1000};do grep ^MemFree /proc/meminfo;done >/dev/null

real    0m0.914s
user    0m0.032s
sys     0m0.148s


cat /proc/meminfo > file
time for i in {1..1000};do grep ^MemFree file;done >/dev/null

real    0m0.938s
user    0m0.032s
sys     0m0.152s


time for i in {1..1000};do echo "$a"|grep ^MemFree; done >/dev/null

real    0m1.016s
user    0m0.040s
sys     0m0.232s