dynamic case statement creation

Question

I have a rather long case statement (about 150 patterns to match).

case "$line" in
  pattern1) ret=result1;;
  pattern2) ret=result2;;
  pattern3) ret=result3;;
   ...
   ...
esac

However, this is not really appropriate as the patterns are really user modifiable data .. So everytime a new pattern is needed, I need to go into and modify the case statement.

What i would like to do is create a separate text file something like:

pattern1=result1
pattern2=result2
pattern3=result3
...
...

The text file would be read and used to create the case statement. This has the advantage of not needing to modify the code every time a new pattern is required.

Is there a way to automatically create a case statement from input from a text file? I'm open to other options, but am looking for a solution that can do the pattern matching to the input line very fast as I also have a large number of input lines to match against.

Answer 1

It's a fairly odd thing to do, dynamic case statement generation...but yes, it's possible.

I would do it something like as follows (if I had explored all other approaches I could think of; I don't know that I would recommend this):

. <( awk -F= 'BEGIN { print "case \"$line\" in" }
                    { print $1 ") ret=" $2 ";;" }
              END   { print "esac" }' patternfile )

If patternfile contains

pattern1=result1
pattern2=result2
pattern3=result3

the awk command by itself would output

case "$line" in
pattern1) ret=result1;;
pattern2) ret=result2;;
pattern3) ret=result3;;
esac

and with the . <( awk ... ) construct this will be sourced into your script just as though you had written the case switch directly in your script file.

---

That answers "dynamic case statement creation" in a shell script. However, since you're planning on doing this within a loop, the above would be a very bad way to do it—since you would run awk 10,000 times on your patternfile, thus outweighing any possible performance benefit.

Instead it would be better to write the code generator to create a case switch within a function definition—create and source a whole function definition, in other words:

# At the top of your script file:
. <( awk -F= 'BEGIN { print "my_case_switch_function() {"
                      print "case \"$1\" in" }
                    { print $1 ") ret=" $2 ";;" }
              END   { print "esac"
                      print "}"   }' patternfile )

# Within your while loop (or wherever else you want):
my_case_switch_function "$line"

This would allow you to re-use the generated case switch over and over (10,000 times if you like) after only processing patternfile once. So the performance is as good as you would get if you manually created the case switch from the patternfile and hard-coded it in your script (except for the small overhead of running awk once on a 150 line file, which is negligible next to processing 10,000 lines).

---

However, it must be reiterated: shell script case switches are not the tool for processing a 10,000 line file line by line. So even though this solution closely approaches the performance you could get with a hardcoded case switch, it will probably still be slow.

To quote Stephane Chazelas:

As said earlier, running one command has a cost. A huge cost if that command is not builtin, but even if they are builtin, the cost is big.

And shells have not been designed to run like that, they have no pretension to being performant programming languages. They are not, they're just command line interpreters. So, little optimisation has been done on this front.

Answer 2

I think this is an XY problem. You don't really need to generate a dynamic case statement. You just want to use a file as a simple key-value store. One solution is to search through the file with grep and then extract the value with cut:

ret=$(grep "^$line" file.txt | cut -d = -f 2)

Answer 3

With this pattern file:

a*c=abc
def=def

and assuming that there is only one = per line:

#! /bin/bash

line=abc
while IFS= read -r patternline; do
        [[ $patternline =~ .=. ]] || continue
        pattern="${patternline%=*}"
        result="${patternline##*=}"
        if [[ "$line" == $pattern ]]; then
                ret="$result"
                echo "$patternline"
                break
        fi
done <patterns.txt

If many lines have to be checked then the file content would be read into an array, though:

#! /bin/bash

patterns_file="patterns.txt"

patterns=()
results=()
patternindex=0


### BEGIN: init
while IFS= read -r patternline; do
        [[ $patternline =~ .=. ]] || continue
        patterns[patternindex]="${patternline%=*}"
        results[patternindex]="${patternline##*=}"
        ((patternindex++))
done <"$patterns_file"
pattern_count="$patternindex"
### END:   init

patterncheck () {
        local line="$1" i=0 pattern= result=
        for((i=0;i<pattern_count;i++)); do
                pattern="${patterns[i]}"
                result="${results[i]}"
                if [[ "$line" == $pattern ]]; then
                        ret="$result"
                        echo "$line"
                        break
                fi
        done
} # patterncheck ()

# tests

patterncheck abc
patterncheck def