IF IN SHELL SCRIPT

$ cat > elf # #!/bin/sh # Script to test if..elif...else # if [ $1 -gt 0 ]; then   echo "$1 is positive" elif [ $1 -lt 0 ] then   echo "$1 is negative" elif [ $1 -eq 0 ] then   echo "$1 is zero" else   echo "Opps! $1 is not number, give number" fi

Try above script as follows:
$ chmod 755 elf
$ ./elf 1
$ ./elf -2
$ ./elf 0
$ ./elf aHere o/p for last sample run:
./elf: [: -gt: unary operator expected
./elf: [: -lt: unary operator expected
./elf: [: -eq: unary operator expected
Opps! a is not number, give number
Above program gives error for last run, here integer comparison is expected therefore error like "./elf: [: -gt: unary operator expected" occurs, but still our program notify this error to user by providing message "Opps! a is not number, give number".