How do I use shell variables in Makefile actions?

Question

I have the following in a Makefile for the purpose of recreating my database including destroying it if necessary. It does not work. ``` .PHONY: rebuilddb exists=$(psql postgres --tuples-only --no-align --command "SELECT 1 FROM pg_database WHERE datname='the_db'") if [ $(exists) -eq 1 ]; then dropdb the_db fi createdb -E UTF8 the_db ``` Running it results in an error: ``` $ make rebuilddb exists= if [ -eq 1 ]; then /bin/sh: -c: line 1: syntax error: unexpected end of file make: *** [rebuilddb_postgres] Error 2 ``` Why is this wrong? It looks like valid Bash as far as I can tell? Are there special considerations I must make when doing this in a Makefile? UPDATE: Using the answer I arrived at a working version: ``` .PHONY: rebuilddb exists=$$(psql postgres --tuples-only --no-align --command "SELECT 1 FROM pg_database WHERE datname='the_db'"); \ if [ "$$exists" == "1" ]; then \ dropdb the_db; \ fi; createdb -E UTF8 the_db ```

Accepted Answer

There are at least two considerations. `$()` references a Make variable. You must escape the `$` to do command substitution. Also, the shell commands must be all on one line. Try:

```
exists=$$(psql postgres --tuples-only --no-align --command "SELECT 1 FROM \
    pg_database WHERE datname='the_db'"); \
    if [ "$$exists" -eq 1 ]; then \
        dropdb the_db; \
    fi; \
    createdb -E UTF8 the_db
```

On the other hand, it seems like it would be simpler to just always try to drop the database, and allow failure:

```
rebuilddb:
    -dropdb the_db  # Leading - instructs make to not abort on error
    createdb -E UTF8 the_db
```

How do I use shell variables in Makefile actions?

Solution

Problem