Add a switch command to pgenv (#53)

Add `switch` command.

The switch command acts mostly like use, however, it does
not manage any database for you, it simply changes the version
linked and thus in the path of the user.
It is implemented as a subcase of `use` that will not start the selected instance.
Documentation updated.

README.md

@@ -14,6 +14,9 @@ Synopsis
 
     # Build PostgreSQL server
     pgenv build 10.4
+
+    # Switch PostgreSQL version
+    pgenv switch 10.4
 
     # Use PostgreSQL version
     pgenv use 10.4
@@ -234,6 +237,20 @@ pgenv use latest 10
 
 will select the most recent PostgreSQL version of the 10 series installed.    
 
+### pgenv switch
+
+Sets the version of PostgreSQL to be used in all shells by symlinking its
+directory to `$PGENV_ROOT/pgsql`. Contrary to `pgenv use` this command does
+_not_ manage a database for you. Meaning, it will not start, stop and
+initialize a postgres database with the given version. Instead it simply
+changes the environment to a different version of PostgreSQL. This can be
+useful if the user has other tools to automate the provisioning and lifecycle
+of a database.
+
+```
+$ pgenv switch 10.4
+```
+
 ### pgenv versions
 
 Lists all PostgreSQL versions known to `pgenv`, and shows an asterisk next to
@@ -571,10 +588,11 @@ Usage: pgenv <command> [<args>]
 
 The pgenv commands are:
     use        Set and start the current PostgreSQL version
-    clear      Stop and unset the current PostgreSQL version
     start      Start the current PostgreSQL server
     stop       Stop the current PostgreSQL server
     restart    Restart the current PostgreSQL server
+    switch     Set the current PostgreSQL version
+    clear      Stop and unset the current PostgreSQL version
     build      Build a specific version of PostgreSQL
     rebuild    Re-build a specific version of PostgreSQL
     remove     Remove a specific version of PostgreSQL

bin/pgenv

@@ -143,10 +143,11 @@ Usage: pgenv <command> [<args>]
 
 The pgenv commands are:
     use        Set and start the current PostgreSQL version
-    clear      Stop and unset the current PostgreSQL version
     start      Start the current PostgreSQL server
     stop       Stop the current PostgreSQL server
     restart    Restart the current PostgreSQL server
+    switch     Set the current PostgreSQL version
+    clear      Stop and unset the current PostgreSQL version
     build      Build a specific version of PostgreSQL
     rebuild    Re-build a specific version of PostgreSQL
     remove     Remove a specific version of PostgreSQL
@@ -1034,7 +1035,10 @@ pgenv_patch_source_tree() {
 }
 
 case $1 in
-    use)
+    use|switch)
+        # save the 'command' to check if we should initialize and start the database or not
+        command=$1
+
         v=$( pgenv_guess_postgresql_version $2 $3)
         pgenv_input_version_or_exit "$v" 'show-installed-versions' 'build'
         pgenv_installed_version_or_exit $v
@@ -1057,11 +1061,15 @@ case $1 in
             ln -nsf pgsql-$v pgsql
         fi
 
-        # Init if needed.
-        pgenv_initdb
+        # only start the database if the user specified the need to use the database
+        if [ "$command" == 'use' ]; then
+            # Init if needed.
+            pgenv_initdb
+
+            # start the instance
+            pgenv_start_instance
+        fi
 
-        # start the instance
-        pgenv_start_instance
         exit
         ;;