Shell Library Design

Library Guard

Load Once

Shell libraries often protect setup code with a guard flag. The guard makes repeated loading predictable by running initialization only once.

Program

Play the script to choose the guard state and see whether setup runs.

library_guard_flag.sh 
#!/usr/bin/env bash

library_loaded=
if [[ "$library_loaded" -eq 0 ]]; then
    library_loaded=1
    status="loaded"
else
    status="already-loaded"
fi
echo "$status"

#!/usr/bin/env bash

library_loaded=
if [[ "$library_loaded" -eq 0 ]]; then
    library_loaded=1
    status="loaded"
else
    status="already-loaded"
fi
echo "$status"
guard flag A guard flag records whether setup has already run.
idempotent load Idempotent loading means repeated loads do not repeat setup work.
library setup Library setup should be explicit and easy to skip when it has already happened.