Shell Programming and Scripting

printf '%s\n' '/^COL2=/s/=.*/=NEWVALUE/' w q | ed -s file

VAR=COL2 VAL=1 sed  -i "s/^${VAR}=.*/${VAR}=${VAL}/" test.txt>tmp
mv tmp test.txt

$cat test.txt 
COL1=TEST 
COL2=1

$ cat test.txt
COL1=TEST
COL2=
COL3=AADSDFSDFDSFDFDF

$ cat test.sh
VAR=COL2
VAL=1
sed "s/^$VAR=.*/$VAR=$VAL/" test.txt

$ ./test.sh
COL1=TEST
COL2=1
COL3=AADSDFSDFDSFDFDF

$ sed "s/COL2=.*/COL2=1/" test.txt
COL1=TEST
COL2=1
COL3=AADSDFSDFDSFDFDF