How to write if else in one line in shell?

Space -- the final frontier. This works:

if [ $SERVICESTATEID -eq 2 ]; then echo "CRITICAL"; else echo "OK"; fi

Note spaces after [ and before ] -- [ is a command name! And I removed an extra $ at the end of $SERVICESTATEID.

An alternative is to spell out test. Then you don't need the final ], which is what I prefer:

if test $SERVICESTATEID -eq 2; then echo "CRITICAL"; else echo "OK"; fi

shell unix if-statement

Write like this, space is required before and after [ and ] in shell

if [ 2 -eq 3 ]; then echo "CRITICAL"; else echo "OK"; fi

shell unix if-statement

Shorter format.

( [ 2 -eq 3 ] && echo "CRITICAL" ) || echo "OK"

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