Because they are strings, you can compare multiple concatenated strings against one another. Balancing values makes for simple means of handling multiple shell tests at once.
[ "${num#90.}${num%[!0]*}" = "90${num%[!0]*}" ] && do_it But that only works if $num is definitely a number. You can validate an integer value for $num like:
[ "$num" -eq "$num" ] Or a float like:
[ "${#num}${num#*.}" -ne "${num%.*}${#num}" ] But case is usually best...
case ".${num##*[!-.0-9]*}" in (.|.[!9]*|.9[!0]*|.90.*[!0]*) ! :;;esac Where outside utilities are concerned, I usually prefer dc to bc because it can be used to execute arbitrary system commands:
dc -e '[!echo execute arbitrary system command]s=' \ -e '90 90.0 ==' Everything within the [] brackets is a string that is saved to the array named = and which is executed as a dc command if the top two values on the main stack (here 90 and 90.0) = one another. The first ! operator within the string is ! operator which executes as a system command all that follows it. You can also read from input for more dc script to execute conditionally with the ? operator.
The -expression syntax is a GNUism, though. You can get the same effect portably with a heredoc or echoing the commands over a pipe to its stdin.
For example:
! dc -e '[!kill -PIPE "$PPID"]s= 90 90.0 ==' && do_it